Text preview for : AN8038.pdf part of Panasonic AN8038 AC-DC switching power supply control IC
with RCC local resonance circuit for
improved conformance with
energy conservation laws
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Voltage Regulators
AN8038, AN8038S
AC-DC switching power supply control IC with RCC local resonance circuit for improved conformance with energy conservation laws
s Overview
The AN8038 and AN8038S are self-excited AC-DC switching power supply control IC that adopt RCC local resonance control. These ICs are designed to achieve high efficiency over a wide range of loads (light loads at the standby mode to heavy loads) for improved conformance with energy conservation laws, and support input levels used worldwide. They are particularly appropriate for use in AV and OA equipments. AN8038
Unit: mm
0.5±0.1
0.51 min. 3.8±0.25 (3.45)
1
8 7
9.4±0.3
2 3 4 6.3±0.3
5
3° to 15°
7.62±0.25
0.150.05
+0.1
DIP008-P-0300B
s Features
AN8038S 5.01±0.20 · Supports improved conformance with energy conser8 5 vation laws by providing two operating modes. With external resistors, it is possible to set the operating point at which the modes change over according to the load power, as shown below. 1.Continuous (RCC) mode 1 4 High efficiency achieved with local resonance operation (zero cross detection). 2.Discontinuous mode (standby) mode Reduced switching loss and standby power due to re(0.60) 1.27 0.40 duced frequency 0.10 Seating plane · Input voltage correction function. This function corrects SOP008-P-0225C the maximum on-period in a manner inversely proportional to the input voltage. · Built-in overvoltage protection function (detects at VCC pin) · Pulse-by-pulse overcurrent protection function (single detection per one cycle) · Packages: 8-pin DIP ··· AN8038 8-pin SOP ··· AN8038S
4.30±0.20 6.40±0.20
0.15±0.10
+0.10 0.05
Unit: mm
(1.05)
0° to10° 0.50±0.20
s Applications
· Facsimiles and other OA equipment · Printers and other personal computer peripheral equipment · AV equipment
1.45±0.10 1.75 max.
2.54
6
1.2±0.25
1
AN8038, AN8038S
s Block Diagram
Voltage Regulators
OVP OVP R S (SD latch) Q INIT
8
VCC
Start Stop
VREF 7V
7V Current reviser (IFB) 2 17.5 k 17.5 k Q Q Q VFB TON 4.2 V High-side clamp TR 1 Low-side clamp Current reviser (ITR) RQ S (TR latch) 0.25 V
CF 3
Q Q
In In
7 6
Out GND
RSTB
Out drive
CF latch
IFB
4
I/V conv.
Q S R R - 0.2 V 5 CLM
TOFF 1.0 V
2
Voltage Regulators
s Pin Descriptions
Pin No. Symbol 1 TR Description
AN8038, AN8038S
Transformer reset. When a transformer reset is detected, i.e., a low level is input to this pin, the IC output goes high. However, the transformer reset signal is ignored during the minimum off-period determined by the CF pin. The maximum on-period is also corrected according to the current flowing out of this pin. Adjusts the light-load detection level that determines the when the IC switches from RCC to discontinuous operation. When the voltage (VFB) which is I-V conversion of current feedback signal from IFB pin goes up higher than this pin, minimum off-period current at CF pin decreases, and operating frequency decreases. The detection level can be adjusted arbitrarily using an external pull-down resistor. Connection for the capacitor that determines the on and off periods for the IC output (Out). Input for the current feedback signal from the power supply output photocoupler Input of the pulse-by-pulse overcurrent protection circuit. Normally, it will be necessary to add an external filter for this input. Ground Output to drive a power MOSFET directly Power supply. This pin watches VCC , and has operating threshold voltages for the start, stop, OVP, and OVP reset levels.
2
RSTB
3 4 5 6 7 8
CF IFB CLM GND VOUT VCC
s Absolute Maximum Ratings
Parameter Supply voltage Peak output current Power dissipation (Independent IC without a heat sink) AN8038S AN8038 Symbol VCC IOP PD (Ta = 25°C) PD (Ta = 85°C) PD (Ta = 25°C) PD (Ta = 85°C) Operating temperature Storage temperature Topr Tstg Rating 28 -1, +2 500 260 306 *1 122 *2 -30 to +85 -55 to +150 °C °C Unit V A mW
Note) *1: When mounted on a printed circuit board: 477 mW *2: When mounted on a printed circuit board: 191 mW
s Recommended Operating Range
Parameter Supply voltage Symbol VCC Range From the stop voltage to the OVP operating voltage Unit V
3
AN8038, AN8038S
s Electrical Characteristics at VCC = 18 V, Ta = 25°C
Parameter Low voltage protection (U.V.L.O.) initial startup supply voltage. Low voltage protection (U.V.L.O.) operation stop supply voltage Overvoltage protection (OVP) operating supply voltage Overvoltage protection (OVP) release supply voltage Overvoltage protection (OVP) operating time circuit current 1 Overvoltage protection (OVP) operating time circuit current 2 Transformer reset detection (TR) threshold voltage Transformer reset detection (TR) upper limit clamp voltage Transformer reset detection (TR) lower limit clamp voltage Transformer reset detection (TR) pin source current Overcurrent protection (CLM) threshold voltage Oscillator (CF) maximum on-period current gain Oscillator (CF) maximum on-period current Oscillator (CF) minimum off-period current 1 Oscillator (CF) minimum off-period current 2 Output oscillator frequency Standby pin (RSTB) voltage Standby operation (RSTB) threshold voltage Current feedback pin (IFB) voltage Pre-startup low-level output voltage Low-level output voltage High-level output voltage Circuit current during startup 1 Circuit current 1 Circuit current 2 4 Symbol Start VCC Stop VCC OVP VCC OVPC VCC OVP ICC1 OVP ICC2 TR VTH TR VCLH TR VCLL TR ITR CLM VTH CF GION CF ION CF IOFF1 CF IOFF2 fOSC VRSTB RSTB VTH VIFB STB VOL VOL VOH ISTART1 OPR1 ICC1 OPR2 ICC2 VCC = 10 V VCC = 18 V IIFB = - 0.7 mA VCC = 12 V IOUT = 0.2 A IOUT = - 0.1 A IFB = Open ITR = 0 mA IIFB = - 0.7 mA IIFB = -1.3 mA CF = 1 000 pF, ITR = -450 µA IIFB = - 0.5 mA ITR = 1 mA ITR = -1 mA VTR = 0.5 V VCC = 22 V 10 V VCC = 22 V 18 V Conditions
Voltage Regulators
Min 12.9 8.0 18.7 6.6 0.4 1.3 0.15 1.2
Typ 14.4 8.9 20.5 7.5 0.53 1.7 0.25 1.5
Max 15.9 9.8 22.3 8.4 0.66 2.1 0.35 1.8 0 -185 1.2 350
Unit V V V V mA mA V V V µA mV µA µA µA kHz V V V V V V µA mA mA
- 0.3 - 0.15 -5 -225 0.8 210 0 -205 1.0 280
-1 560 -1 250 -940 -70 105 3.2 0.3 5 15.5 120 6.1 6.4 -55 140 3.5 0.5 5.6 1.0 0.9 16.3 190 8.7 9.1 -40 175 3.8 0.7 6.2 1.25 2.0 280 11.3 11.8
Voltage Regulators
s Electrical Characteristics at VCC = 18 V, Ta = 25°C (continued)
· Design Reference Data
AN8038, AN8038S
Note) The characteristics listed below are theoretical values based on the IC design and are not guaranteed.
Parameter Low-voltage protection (U.V.L.O.) start/stop supply voltage difference Transformer reset (TR) detection threshold hysteresis width Oscillator (CF) upper limit voltage Oscillator (CF) lower limit voltage 1 Oscillator (CF) lower limit voltage 2 Maximum on-period Minimum off-period 1 Minimum off-period 2 Light-load oscillator frequency Output rise time Output fall time TR output response time CLM output response time Circuit current during startup 2
Symbol VCC TR VTH VCLH VCFL1 VCFL2 tON(max) tOFF(min)1 tOFF(min)2 fOSC2 tr tf tTR tCLM ISTART2
Conditions
Min
Typ 5.4 0.1 4.2 1.0 0.1 12 2.6 67 15 40 20 400 100 190
Max 300
Unit V V V V V µs µs µs kHz ns ns ns ns µA
IIFB = - 0.5 mA, CF = 1 000 pF IIFB = - 0.5 mA, CF = 1 000 pF IIFB = - 0.2 mA, CF = 1 000 pF IIFB = - 0.2 mA, CF = 1 000 pF VTR = 0.1 V IIFB = - 0.2 mA, CF = 1 000 pF VTR = 0.1 V IIFB = -1.3 mA, CF = 1 000 pF ITR = -450 µA IIFB = -1.3 mA, CF = 1 000 pF ITR = -450 µA 10% to 90%, IOUT = 0 mA 10% to 90%, IOUT = 0 mA
Ta = -30°C to +85°C
100
s Terminal Equivalent Circuits
Pin No. 1 Equivalent Circuit Description TR: Transformer reset detection input. When a transformer reset is detected, i.e., a low level is input to this pin, the IC output goes high. However, the transformer reset signal is ignored if the signal is shorter than the minimum offperiod determined by the CF pin. Also note that the maximum on-period is corrected according to the source current. I/O I
VREF 7V TR 1 High-side Low-side clamp clamp
5
AN8038, AN8038S
s Terminal Equivalent Circuits (continued)
Pin No. 2 VREF 17.5 k
Comp.
Voltage Regulators
Equivalent Circuit
Description RSTB: Adjusts the light-load detection level that determines the time when the IC switches from RCC to discontinuous operation. When the voltage which is I-V conversion of current feedback signal goes up higher than this pin, operating frequency is reduced. An arbitrary level can be set by inserting an external pull-down resistor. CF: Connection for the capacitor that determines the on- and off-periods of the IC output (Out). The on- and off-periods are corrected by ION which is proportional to the flowing out current at the TR pin, and IOFF which corresponds to the current at IFB pin. IFB: Connection for the photocoupler used for the power supply output error-voltage feedback. This input can decrease the photocoupler dark current by about 250 µA.
I/O
RSTB 2 CF 17.5 k
3
VREF IOFF
Comp.
VFB 3 CF 4 ION
VREF ×1 VFB 20 k ×5 1 k 4 IFB
I
5
VREF
Comp.
CLM: Input to the pulse-by-pulse overcurrent protection circuit. Normally, we recommend adding an external filter for this input.
I
5 CLM
6
GND 6
GND: IC ground.
6
Voltage Regulators
s Terminal Equivalent Circuits (continued)
Pin No. 7 Equivalent Circuit
AN8038, AN8038S
Description VOUT: Output used to directly drive a power MOSFET. A totem pole structure is adopted in this output circuit. The absolute maximum ratings for the output current are: Peak: +2 A, -1 A DC: +200 mA, -100 mA
I/O I
VCC
7 VOUT
8
VCC 8
VCC: Power supply. This pin monitors supply voltage and has the threshold for the start, stop, OVP, and OVP reset levels.
s Usage Notes
The circuit current during startup is set to a low level to minimize power loss due to the startup resistor. However, VCC ripple caused by the power transistor switching on and off may result in incorrect operation of the U.V.L.O. circuit and failure to start. The figure shows the allowable range for VCC ripple. Insert a capacitor near the IC's VCC and GND pins to reduce VCC ripple so that it remains within the allowable range. Allowable VCC ripple range
3
2.5
VCC ripple amplitude (V[p-p])
2
1.5
Recommended operating range
1
0.6 0.5
0
10
70 100
300
1 000
10 000
VCC ripple frequency (kHz)
7
AN8038, AN8038S
s Application Notes
[1] Timing charts · Circuit diagram (E) (B)
Voltage Regulators
(F)
TR
CF
VCC Out
(A) (D)
CLM
(C) (G)
· Normal control waveforms
(A) Bias winding voltage 0V Off On Off On Off On Off
(B) TR pin voltage
0V Input voltage correction EIN: large, tON: short tON cannot be accepted during tOFF(min). VFB voltage
(C) CF pin voltage 4.2 V VRSTB Can be modified with an external resistor.
0V IFB correction VFB: large, tOFF(min): long
(D) Out pin voltage VCC -1.5 V Off 0V On Off On Off
On
Off
Winding current (F) Secondary winding current 0A Heavy
(E) Primary winding current
Load Continuous (RCC) mode
Light Discontinuous mode
8
Voltage Regulators
s Application Notes (continued)
[1] Timing charts (continued) · During pulse-by-pulse overcurrent operation
AN8038, AN8038S
(A) Bias winding voltage 0V Off On Off On Off On Off On
(B) TR pin voltage 0V
(C) CF pin voltage 4.2 V VRSTB Can be modified with an external resistor.
VFB voltage 0V
Rapid charging during CLM operation
(D) Out pin voltage VCC -1.5 V Off 0V (E) Winding current Primary winding current Secondary winding current On-period limitation during CLM operation On Off On Off On Off On
0A Light (G) CLM pin voltage 0V
Load
Heavy
-205 mV CLM threshold voltage Overcurrent detection
9
AN8038, AN8038S
s Application Notes (continued)
[2] Operation descriptions 1. Start/stop circuit block · Startup mechanism After the AC voltage is applied and the supply voltage due to the current in the startup resistor reaches the startup voltage and the IC begins to operate, drive of the power MOSFET begins. This causes a bias in the transformer, and the supply voltage is provided to the IC from the bias winding. (This is point a in figure 1.) During the period between the point when the startup voltage is reached, and the point when the bias winding can generate a voltage enough to supply the IC, the IC supply voltage is provided by the capacitor (C8) connected to VCC . Since the supply voltage falls during this period (area b in figure 1), if the supply voltage falls below the IC stopvoltage before an adequate supply voltage can be provided by the bias winding, it will not be possible to start the power supply. (This is the state at point c in figure 1.)
After AC rectification
Voltage Regulators
Startup resistor R1
VCC C8
GND
VOUT
Before startup Startup voltage Stop voltage b a
Startup Voltage supplied from the bias winding Startup state c Startup failure
Figure 1 · Functions This IC includes a function that monitors the VCC voltage. It starts IC operation when VCC reaches the startup voltage (14.4 V typical), and stops operation when the voltage falls below the stop voltage (8.9 V typical). Since a large voltage difference (5.5 V typical) is taken between the start and stop voltages, it is easy to select values for the start resistor and the capacitor connected to VCC .
Since high voltages are applied across the startup resistor, measures must be taken to minimize the current that flows in this resistor. (To use a smaller startup resistor.) To achieve this, the circuit current at startup is set to as small as 190 µA (typical), and temperature variations, and also sample-to-sample variations are reduced as well. Since the bias current is reduced, the capacitor connected to VCC can be miniaturized as well. 2. Oscillator circuit The oscillator circuit determines the pulse width with which the main switch is turned on and off using the charge and discharge of the capacitor CCF connected to the CF pin (pin 3). This IC implements a control scheme in which the main switch on-period is the discharge period of the CF pin waveform, and the off-period is the charge period of that waveform. Constant-voltage control in a switching power supply using this IC is implemented during RCC (continuous) operation by holding the main switch off-period fixed and varying the on-period. This on-period is controlled by directly varying the output pulse-width of the oscillator circuit. Additionally, the IC reduces the maximum on-period when the input voltage increases by detecting the input voltage through the flowing out current at the TR pin. (See figure 2.) Furthermore, this IC features an added function that detects increases of the IFB feedback current, and reduces the off period, and lowers the operating frequency to reduce power loss during standby (light load) mode and to prevent being out of control. During overcurrent protection operation, the IC performs a rapid discharge operation where the CLM pin voltage reaches the threshold voltage of -205 mV (typical).
10
Voltage Regulators
s Application Notes (continued)
[2] Operation descriptions (continued) 2. Oscillator circuit (continued)
AN8038, AN8038S
tON is reduced since ITR increases when EIN is large.
ITR
RTR2 RTR1
EIN+
NP
NB TR IFB Current mirror 5:1 20 k Current mirror 1:1 ITR = ION ITR ION
CF
VCC
Out
PC
VFB 280 µA typ.
GND
CCF
EIN-
CF pin voltage 4.2 V Control switching voltage VRSTB Output monitoring voltage VFB
Input voltage correction EIN: larger On-period: shorter
0V VCC - 1.5 V Out pin voltage 0V On-period Off-period
Figure 2. On-period block diagram and control waveforms
11
AN8038, AN8038S
s Application Notes (continued)
[2] Operation descriptions (continued)
Voltage Regulators
2. Oscillator circuit (continued) · Notes on switching from RCC operation to discontinuous operation When the state changes from normal load to standby mode, the post-I-V conversion output monitor voltage VFB increases along with the increase of the amount of feedback current. Then, when the IFB pin feedback current exceeds 1 mA (typical) and the VFB voltage becomes larger than the RSTB pin (pin 2) voltage, the minimum offperiod current that is from the CF pin lower limit value until the RSTB pin voltage is reached, is rapidly reduced, and the operating frequency is lowered. (See figure 3.) This allows the switching loss to be reduced to the minimum, and allows the standby mode power to be reduced. Furthermore, the RSTB pin voltage, which is operating point for switching from RCC operation to discontinuous operation, is set by resistor-division of the internal 7 V reference voltage to a typical value of 3.5 V. This value can be adjusted by connecting an external pull-down resistor, and this can be used to suppress increases of the minimum off-period. Note that application designs must take into account the sample-to-sample variations of ±15% in the internal resistors and temperature coefficient of 2 400 ppm/°C.
Allows the control switching operating point 17.5 k to be lowered. RSTB 17.5 k 7V Comp. 7V NB VCC Out EIN+ NP
IFB PC
Current mirror 5:1 VFB 20 k IOFF = 1250 µA typ. (IIFB < 1 mA) IOFF = 55 µA typ. (IIFB > 1 mA)
CF
GND IOFF
EIN- CCF
a) Normal control - heavy load (IIFB < 1 mA) 4.2 V CF pin voltage VRSTB VFB 0.2 V 0V VCC -1.5 V Out pin voltage 0V tOFF(min) tOFF On 0.2 V 0V VCC -1.5 V Out pin voltage 0V 4.2 V CF pin voltage VRSTB
b) Standby (IIFB > 1 mA)
VFB
tOFF(min) tOFF
The minimum off-period current is reduced when VFB VRSTB . tOFF (min): large On
Figure 3. Off-period block diagram and control waveforms
12
Voltage Regulators
s Application Notes (continued)
[2] Operation descriptions (continued)
AN8038, AN8038S
2. Oscillator circuit (continued) · Notes on setting the oscillator frequency This section describes the calculation of the on- and off-periods. 1) On-period: tON The output on-period is the discharge period when the CF pin is between the peak value of VCFH = 4.2 V (typical) and VFB . The following formula can be used to calculate the on-period of the power MOSFET as a reasonable approximation. (See figure 2.) tON = CCF × (VCFH - VFB) / ION Here, VCFH = 4.2 V typ. ION = ITR + 280 µA typ. ITR = (EIN × NB/NP) / (RTR1 + RTR2) VFB = 0.7 V typ. (When IIFB 250 µA) VFB = 4 k × (IIFB - 250 µA) + 0.7 V (When IIFB > 250 µA)
2) Off-period: tOFF The minimum off-period is the charging period from VCFL = 0.2 V (typical) to VRSTB . The following formula can be used to calculate the minimum off-period as a reasonable approximation. (See figure 3.) tOFF(min) = CCF × {VRSTB - VCFL} / IOFF IOFF1 = 1250 µA typ. (VFB 0.7 V typ. to VRSTB) IOFF2 = 55 µA typ. (VFB VRSTB to 4.2 V typ.) tON tOFF(min) CCF VCFH VCFL VRSTB : On-period : Minimum off-period : Capacitance of the capacitor connected to the CF pin : Upper limit voltage for the CF pin. VCFH = 4.2 V typ. : Lower limit voltage for the CF pin : Pin voltage that sets the light load detection level. VRSTB = 3.5 V typ. This voltage can be adjusted with an external resistor. VFB : Value that results from IC internal conversion of feedback current IIFB ION : On-period (CF pin discharge) current IOFF1 : Off-period (CF pin charge) current when IIFB < 1 mA IOFF2 : Off-period (CF pin charge) current when IIFB > 1 mA ITR : Flowing current at TR pin EIN : Primary winding voltage NB : Number of turns in the bias winding NP : Number of turns in the primary winding RTR1/RTR2 : Resistors connected to the TR pin
However, the off-period for the local resonance circuit described later, is determined by the time for the voltage fed back to TR pin (pin 1) if that time is longer than the tOFF time determined by CCF . The power MOSFET is turned on and off continuously by repeating the above operations.
13
AN8038, AN8038S
s Application Notes (continued)
[2] Operation descriptions (continued)
Voltage Regulators
3. Local resonance operation (power MOSFET turn on delay circuit) The AN8038 and AN8038S transformer reset detection functions detect a low-level input to the TR pin. (This is similar to earlier Panasonic ICs, in particular the AN8026, AN8027, AN8028, and AN8029.) Local resonance operation is possible with circuits as shown in figure 4. C7 is the resonance capacitor, and R9 and C9 form a delay circuit for adjusting the power MOSFET turn on time. When the power MOSFET is off, the voltage that occurs in the drive winding is input to the TR pin (pin 1) through R9 and C9. The power MOSFET will be held in the off state while a high level (a level higher than the threshold voltage, which is 0.25 V typical) is input to the TR pin. The TR pin also has a clamping capability for upper and lower limit voltages. The upper limit voltage is clamped at 1.5 V typical (sink current is -3 mA), and the lower limit voltage is clamped at about - 0.15 V typical (source current: 3 mA). (See figure 5.) The power MOSFET off-period is determined by the longer period of the following two periods: the period until the TR pin input voltage becomes lower than the threshold voltage as the bias winding voltage falls after the transformer discharges its energy, and the minimum off-period tOFF(min) stipulated by the internal oscillator (see the description of the oscillator circuit). As a result, ringing in the bias winding does not be regarded as a turn on signal during the minimum off-period.
EIN+ Startup resistor R1 DB
Lower limit clamp current
NP N S
VO
VCC Lower limit clamp Transformer reset - 0.15 V typ. comparator
C7 TR Out GND
Upper limit clamp current
R9 C9
NB
0.25 V typ.
Upper limit clamp 1.5 V typ.
EIN- Figure 4
14
Voltage Regulators
s Application Notes (continued)
[2] Operation descriptions (continued)
AN8038, AN8038S
3. Local resonance operation (power MOSFET turn on delay circuit) (continued) · Notes on C7 value selection Figure 5 shows the local resonance N VP = P VO waveform. NS tPD Select values of R9 and C9 to deterVDS mine the delay time so that the power VP MOSFET is turned on at the 1/2 cycle point in the resonant frequency waveVIN form. Simply stated, select values so that the power MOSFET turns on at the 0V zero voltage point in the voltage waveform. The resonant frequency can be 1.5 V typ. VTR roughly determined using the follow0.25 V ing formula. 0V 1 - 0.15 V typ. fSYNC = (Hz) 2 L · C7 tTR C7 : Resonance capacitor Out L : Inductance of the transformer primary winding 0V Accordingly, the turn on delay time tPD(ON) to turn on the power MOSFET ID at the 1/2 cycle point in the resonant 0V frequency waveform is given by the following formula. Figure 5. Local resonance waveform tPD(ON) = L · C7 (s) Furthermore, tPD(ON) should be set to be larger than tTR, since a response time tTR that is the time between transistor detection and output changeover is about 400 ns. Note that since insertion of the resonance capacitor C7 results in increased losses, using the parasitic capacitance of the power MOSFET itself should also be considered. However, in this case the sample-tosample variations and temperature variations should be considered. · Notes on R9 and C9 value selection If an excessively low value is used for R9, the current flowing into the TR pin after power supply startup will exceed the maximum rating for the IC, and incorrect operation (or even, in the worst case, destruction of the device) may occur. We recommend using a value of R9 in the range that satisfies the following conditions. VB(-) - The TR lower limit clamp voltage (- 0.15 V typical) -3 mA R9 VB(-): The peak voltage when the voltage is negative VB(+) - The TR upper limit clamp voltage (1.5 V typical) 3 mA R9 VB(+): The peak voltage when the voltage is positive NB NB VB(+) = V VB(-) = - E NS O NP IN Also, adjust tPD(ON) by changing the value of C9 in consideration of the resonance capacitance and inductance of the transformer used.
15
AN8038, AN8038S
s Application Notes (continued)
[2] Operation descriptions (continued)
Voltage Regulators
4. Power supply output control system (IFB: feedback) Constant-voltage control of the power supply output is conducted by changing the on- and off-periods of the power MOSFET. On- and off-periods are controlled as follows: a feedback current responding to the output of output-voltage detection circuit formed at secondary output side is input to IFB pin through a photocoupler connected to IFB pin (pin 14), and is converted to VFB voltage. (See figure 6.) The more AC input voltage becomes higher and/or the more the load current decreases, the more the flowout current from IFB pin increases. This makes VFB voltage higher and on-period shorter (at the standby, off-period becomes longer). Furthermore cancellation ability is about 250 µA for the dark current of photocoupler.
7V 5:1 250 µA typ. VFB 20 k PC
Secondary side power supply output
IFB 4
This transistor discharges the soft start capacitance when the IC stops.
At least 1 mA is required (to bias the AN1431T/M.) AN1431T/M
PC Primary side Secondary side
Figure 6. Power supply output control system
5 4
VFB (V)
3 2
Dark current
1 0 0
- 0.2
- 0.4
- 0.6
- 0.8
-1.0
-1.2
IIFB (mA)
Figure 7. Feedback current vs. VFB characteristics
16
Voltage Regulators
s Application Notes (continued)
[2] Operation descriptions (continued)
AN8038, AN8038S
4. Power supply output control system (IFB: feedback) (continued) · Soft start When a power supply is started, it starts up in an overloaded state due to the capacitor connected to the power supply output. Since the power supply output voltage is low in this state, the normal constant-voltage control would rise the power supply to its maximum duty ratio. Although the pulse-by-pulse overcurrent protection circuit (CLM) would limit the current, due to filter and other delays, it cannot decrease the pulse width to zero, and thus large currents could flow in both the main switch (the power MOSFET) and the secondary side diode. This could result in destruction of these components in the worst case. To prevent this, soft start is used to suppress surge currents at power supply startup. Soft start is installed by inserting R3 and C4 between the IFB pin (pin 4) and the GND pin (pin 6) as shown in figure 8. When the IC supply voltage reaches the startup voltage, and the start circuit operates, an open bias (about 6.4 V) is output to the IFB pin. A charging current (IIFB) flows from the IFB pin into C4 due to this voltage. As a result, since startup begins at relatively high VFB, output control is started from short tON. Since the voltage across C4 rises according to the time constant determined by R3 and C4, IIFB becomes smaller with time, and the tON time increases gradually. Due to the above operation, the current that flows in the power MOSFET at power-on increases gradually. As a result, surge currents are suppressed. However, this reduces the transient response of the feedback loop, so care is required in designing this circuit. Rectified AC Startup resistor R1
VCC C8 IFB R3 C4 Out
GND
PC1
CLM R8 C6
R7 To AC(-) Figure 8 5. Output block This IC adopts a totem pole (push-pull) structure output circuit in which NPN transistors as shown in figure 9 sinks and sources current to rapidly drive the power MOSFET which is a capacitive load. This circuit provides maximum sink and source currents of - 0.1 A and +0.2 A (DC), and peak currents of -1 A and +2 A. Furthermore, this circuit has a sink capability of 1 mA (typical) even when the supply voltage has fallen under the stop voltage, and thus can turn off the power MOSFET reliably. 17
AN8038, AN8038S
s Application Notes (continued)
[2] Operation descriptions (continued)
Voltage Regulators
5. Output block (continued) The main requirement on the control IC in this type of power supply is the ability to provide a large peak current. That is to say, a high average current is not required in steady state operation. This is because the power MOSFET is a capacitive load, and while a large peak current is required to drive such a load rapidly, once the load has been charged or discharged a much smaller current suffices to retain that state. This IC has a guaranteed peak current capability of -1 A and +2 A, values which were determined by considering the capacitance of the power MOSFETs that will be used. The parasitic inductance and capacitance of the power MOSFET can cause ringing, and pull down the output pin below the ground level. If the output pin goes to a negative voltage that is larger than the voltage drop of the diode, this state can turn on the parasitic diode formed by the collector of the output NPN transistor and the substrate. Insert a Schottky barrier diode between the output and ground if this is a problem. (See figure 9.) Figure 9 · Overvoltage protection circuit (OVP) OVP stands for overvoltage protection. The overvoltage protection circuit is a self-diagnostic function that shuts down the power supply to protect the load if a voltage that is significantly and abnormally higher than the normal output voltage occurs in the power supply output, due to, for example, a malfunction in the control system or an abnormal voltage applied externally. (See figure 10.) The overvoltage detection function monitors the VCC pin voltage. Since the VCC pin voltage is normally supplied from the transformer bias winding, this voltage is proportional to the secondary side output voltage. Thus the overvoltage protection circuit operates when an overvoltage occurs in the secondary side output. 1) If, as a result of an abnormality in the power supply output, the voltage input to the VCC pin exceeds the threshold value (20.5 V typical), the IC internal reference voltage is shut down, and all control operation is stopped. The IC then holds this state. 2) The OVP circuit is released (reset) by lowering the supply voltage (VCC < 7.5 V typical). (This is the OVP release supply voltage.)
Since output is stopped after the OVP circuit operates, no current is supplied from the bias winding. Power supply output
Schottky diode
Rectified AC Current supply from the startup resistor continues as long as the power supply input voltage (AC) is applied. RS latch S OVP operating voltage 20.5 V typ. R Q OVP release voltage 7.5 V typ. Startup resistor R1
When an abnormal voltage is applied by some external source.
Figure 10. OVP operating circuit
18
Voltage Regulators
s Application Notes (continued)
[2] Operation descriptions (continued)
AN8038, AN8038S
5. Output block (continued) · Operating power supply current characteristics When the OVP circuit operates and the power supply current drops, this can induce a rise of the supply voltage VCC. In the worst case, it may exceed the IC's guaranteed breakdown voltage (28 V). Therefore, the circuit is provided with characteristics that cause the supply current to rise in constant resistance mode when the OVP circuit operates, and thus the increases of the supply voltage. Due to these characteristics, if the ICC supply voltage VCC when the OVP circuit operates is stabilized at a value (note that 1.7 mA typ. this value depends on the value of the startup resistor) that is larger than the OVP release voltage (7.5 V typical), the OVP 0.53 mA typ. circuit will not be reset as long as the AC input is not cut. (See figure 11.) Note that this does not apply to an external reset. 7.5 V 10 V 18 V VCC Figure 11. OVP operating circuit current · Overcurrent protection circuit (pulse-by-pulse overcurrent protection) This circuit uses the fact that overcurrents in the power supply output are proportional to the current flowing in the primary side main switch (power MOSFET). This circuit limits overcurrents in the power supply output by constraining the upper limit of the pulse current flowing in the main switch, and thus protects components sensitive to excessive current. The current flowing in the main switch is detected by connecting a resistor between the power MOSFET source and ground and monitoring the voltage that appears across that resistor. When the power MOSFET is turned on and the CLM (current limit) threshold voltage is detected, the output is turned off. This controls the circuit so that a current in excess of that limit cannot flow by turning off the power MOSFET. The CLM threshold voltage is about -205 mV with respect to ground at Ta = 25°C. While this control operation is repeated every cycle, once an overcurrent is detected, the off state is held for the remainder of that cycle, and the circuit is not turned on until the next period. This type of overcurrent detection is called "pulse-by-pulse overcurrent detection." R6 and C6 in figure 12 form a filter circuit that rejects noise generated due to the incidental equivalent parasitic capacitance when the power MOSFET is on. For the grounding point, we recommend that the power MOSFET source pin and the IC GND pin be connected over as short a distance as possible.
CLM
R6 R8 C6
Use a capacitor with excellent frequency characteristics.
GND R7 To AC- Use resistors with no inductance.
Figure 12 · Notes on the detection level precision This overcurrent detection level is reflected on the operating current level of the power supply overcurrent protection function. Therefore, if this detection level varies with sample-to-sample variations or with temperature, the operating current level of the overcurrent protection function of the power supply itself will vary. Since variations in this level imply a need for increased ruggedness in parts used, or even the destruction of circuit components, we have increased the precision of this IC as much as possible.
19
AN8038, AN8038S
s Application Notes (continued)
[2] Operation descriptions (continued)
Voltage Regulators
5. Output block (continued) · Overcurrent protection circuit (input voltage correction function) As an extended application, this section presents a circuit design that applies a correction so that the overcurrent protection operating point is held fixed with respect to variations in the input voltage. This circuit uses the proportional relationship between the input voltage and the inverted voltage of the bias winding, and superposes inverted voltage of the bias winding on the overcurrent protection operating voltage. (See figures 13 and 14.) EIN+ VCC VB TR
CLM GND
Drs Out Rrs Zrs
For an FB voltage: cut off For an FF voltage: correction operation IDS
VCLM Input voltage correction Determines the amount of correction EIN- Figure 13
Determines the input voltage at which correction starts.
VDS
0V
VB 0V
FB voltage FF voltage
Input voltage: larger Amount of correction: larger
IDS 0A 0V VCLM
Input voltage EIN
Correction corresponding to the input voltage
Figure 14 20
Voltage Regulators
s Application Notes (continued)
[2] Operation descriptions (continued)
AN8038, AN8038S
6. Notes on feedback control If the IC output pin (pin 7) falls to a negative voltage lower than that of the GND pin, the startup operation may fail or the output oscillation may become unstable. ICs in general, not just this IC, do not respond well when negative voltages lower than the ground level are applied to their pins. (Except for special applications.) This is because parasitic device operations may be induced when negative voltages are applied due to the structure of ICs themselves. In the case mentioned above, when the IC output (VOUT) is turned off, the power MOSFET drain-toRectified AC source voltage, VDS, jumps from a low voltage to a high voltage. The voltage chattering that occurs at this time is superposed on VOUT through the parasitic capacitance CGD between the power MOSFET gate and drain, and generates a negative voltage with respect to the pin. No problems occur if the peak voltage, VEX, CGD of this negative voltage does not exceed the parasitic VDS Out device conduction voltage (about - 0.7 V). However, the amplitude of the chattering is larger for higher input voltages and for larger leakage inductance in the transformer used. Also, the influence Figure 15 of this phenomenon becomes more noticeable for the larger CGD of the power MOSFET used, and the VEX peak value also increases. If the parasitic device conduction voltage is exceeded, then, in this IC, the parts of the circuit around the feedback circuit (FB) (in parVDS ticular, the FB discharge circuit) are influenced. This can cause momentary drops in the IFB pin voltage (the control voltage), and as a result increase the FB current (IIFB) and thus does not allow the drive pulse on-period tON to be increased. It may also prevent VOUT stabilization of the circuit. These are symptoms of the GND case described here. (See figures 15 and 16.) VEX [Countermeasures] Control If an application exhibits the symptoms of the case pin IFB described above, or similar symptoms, first insert a Figure 16 Schottky diode between VOUT and GND. It is not possible to completely remove the mechanism described above from a power supply system. It is also not possible to prevent levels from being pulled down to negative voltages in the control IC itself. Therefore, the most important point in designing countermeasures is to prevent such negative voltages from reaching the parasitic device conduction voltage.
Note) If a Schottky diode is added to the circuit and the condition improves initially but the symptoms reappear when the input voltage or other parameter is increased, try replacing the Schottky diode with one that has a larger forward current (both peak and average values). The current capability of the Schottky diode is sometime insufficient. Reference: Panasonic Schottky Diode Part No. MA2C700A (MA700A) MA2C723 (MA723) MA2C719 (MA719) Note) : Former part number Reverse voltage 30 V 30 V 40 V Forward current (average) 30 mA 200 mA 500 mA Forward current (peak) 150 mA 300 mA 1A
GND
IFB
21
AN8038, AN8038S
s Application Circuit Examples
· Application circuit example 1
Voltage Regulators
EIN+
VOUT
CC
PC AN1432MS
RSTB
2 7 Out
IFB
4
8
6 GND
PC TR 1
CF 3
5 CLM
EIN-
· Application circuit example 2
C1 L1 C2 C3 C4 C12 L2
+
C5 180 µF
-
R1 68 k
D7 MA3F750 (MA750) 15 V/0.4 A C16 1 000 µF D8
R1 68 k
R8 C9 15 k 0.033 µF PC1
R16
D2
MA3D798 (MA10798)
0 MA2C166 (MA166) D11 MA2C166 (MA166) R20 5.1 k D3 MA2C166 (MA166) R5 47 R3 75 k Q2 D9 C6 MA3D760 (MA7D60) FBI
C17 1 000 µF PC1
R10 750 R11 510 C15 1.5 µF AN1431M/T
6.0 V/0.6 A R12 500 R14 1.8 k R13 1.5 k GND
Q3 C10 100 µF 5 CLM 6 GND 8 VCC 7 Out
C18 47 µF -28 V/30 mA
S2 MA2C700 (MA700) C11 1 000 pF 0.015 µF 68 k S1 MA2C700 (MA700)
R4 47 2SK1611 C7 2 200 pF
AN8038, AN8038S
1 2 3 CF TR RSTB FB 4
R7 820 R15 51 R6 0.22
Note) : Former part number
22