AN8028 PANASONIC

Document overview

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Technical content

Self-excited RCC pseudo-resonance type AC-DC switching power supply control IC n Overview The AN8028 is an IC developed for controlling the self-excited switching power supply employing the RCC pseudo-resonance type control method. It is compact, equipped only with the necessary mini- mum functions. The maximum on-period and the minimum off-pe- riod can be set separately by using the external capacitor and resistor respectively. It is suitable for the power supply of AV equipment. n Features

  • Operating supply voltage range: Stop voltage (8.6 V typical) to 34 V
  • Output block employs the totem pole system.
  • Power MOSFET can be directly driven. (output peak current: –1 A maximum)
  • Small pre-start operating current (95 mA typical) allows using a small size start resistor.
  • Built-in pulse-by-pulse overcurrent protection function
  • Incorporating the protection circuit against malfunction at low voltage (start/stop: 14.9 V/8.6 V)
  • Built-in overvoltage protection function (externally resettable)
  • Built-in timer latch function
  • Equipped with frequency (VF) control function.
  • 9-pin single inline package expands the freedom of board design n Applications
  • Televisions, facsimiles, printers, scanners, video equipment n Block Diagram SIP009-P-0000C Unit: mm 0.3+0.1 –0.05 23.3±0.3 6.0±0.3 2.4±0.25 3.3±0.25 1.5±0.25 0.5±0.1 2.541.5±0.25 1.4±0.3 3.0±0.3 30° V REF (7.1 V) U.V.L.O. 8.6V 1.5V Current source 0.1 V CLM TIM Q IFB Switchdiode Q R S RSlatch Signal IN IN V OUT GND5 CLM4TDL 1 FB 9 TIM/OVP 8 V CC TOFF 2 TON 3 0.7 V

0.32 VLow-side

2.8 V

n Absolute Maximum Ratings Parameter Symbol Rating Unit Supply voltage V CC 35 V Peak output current I 6PEAK –1A Power dissipation P D 874 mW Operating ambient temperature * T opr -30 to +85 °C Storage temperature * Tstg -55 to +150 °C Pin No. Symbol Description

1 TDL Transformer reset detection

2 TOFF Pin for connecting C and R to set minimum off-period

3 TON Pin for connecting C to set minimum on-period

4 CLM Input pin for overcurrent protection detection

5 GND Grounding pin

7V CC Power supply voltage pin

8 TIM/OVP Pin for use both overvoltage protection circuit and timer latch

9 FB Photocoupler connection pin for error voltage feedback

Note) *: Expect for the operating ambient temperature and storage temperature, all ratings are for Ta = 25°C. n Recommended Operating Range Parameter Symbol Range Unit Supply voltage V CC From stop voltage to 34 V n Electrical Characteristics at VCC = 18 V, Ta = 25°C Parameter Symbol Conditions Min Typ Max Unit OVP operation threshold voltage V 8OVP 6.1 7.3 8.5 V OVP release voltage V 7OVP 7.4 8.2 9 V OVP operating circuit current 2 I 7OVP2 V CC = 20 V, VOVP = 8.5 V 5.9 7.7 9.5 mA TDL threshold voltage V 1TDL 0.22 0.32 0.42 V TDL upper limit clamp voltage V 1TDL/H ITDL = 3mA 2 2.8 3.6 V TDL lower limit clamp voltage V 1TDL/L ITDL = -3mA - 0.3 0 0.3 V CLM threshold voltage V 4CLM -220 -200 -180 mV

n Electrical Characteristics at VCC = 18 V, Ta = 25°C (continued) Parameter Symbol Conditions Min Typ Max Unit TON maximum on-period current I 3TON FB terminal = open -135 -110 -85 mA TON terminal = GND TON upper limit voltage V 3TON/H FB terminal = open 0.55 0.7 0.85 V TON lower limit voltage V 3TON/L FB terminal = open - 0.1 0.05 0.2 V TOFF upper limit voltage V 2TOFF/H 0.7 0.9 1.1 V TOFF lower limit voltage V 2TOFF/L - 0.1 0.05 0.2 V Output oscillation frequency f OSC C ON = 2 200 pF, ROFF = 1.5 kW 55 65 75 kHz C OFF = 1 000 pF Output current feedback current gain GIFB IFB = -1 mA 5.05 6.8 8.55 ¾ Pre-start low-level output voltage V6STB/L V CC = 10 V, IOUT = 10 mA ¾ 1 1.25 V Low-level output voltage 1 V 6L(1) IOUT = 10 mA ¾ 0.9 2 V Low-level output voltage 2 V 6L(2) IOUT = 100 mA ¾ 1.1 2.2 V High-level output voltage 1 V 6H(1) IOUT = -10 mA 15.7 16.5 ¾ V High-level output voltage 2 V 6H(2) IOUT = -100 mA 15.5 16.3 ¾ V Pre-start circuit current I 7STB V CC = 12V 55 95 135 mA Circuit current 1 I 7OPR(1) V CC = 18 V 8.55 11.5 14.3 mA TON terminal = GND FB terminal = open Circuit current 2 I 7OPR(2) V CC = 34 V 9.6 12.5 15.4 mA TON terminal = GND FB terminal = open TDL flowing-out current I 1TDL V TDL = 0.5 V -50 ¾m A Pin No. Equivalent circuit Description I/O

1 TDL: I

Transformer reset detection terminal. When the transformer reset is detected and low is inputted into the terminal, the output of the IC OUT ) becomes high. However, low-level sig- nal under the minimum off-period determined by the TOFF is ignored.

2 TOFF: ¾

Terminal for connecting the resistor and capaci- tor for determining the minimum off-period (low) of the IC output (V OUT ). An equation for approximate calculation of the minimum off-period (TOFF ) is as follows: TOFF = 2.2 · C · R C: External capacitance R: External resistance n Terminal Equivalent Circuits V REF High- side clamp Low- side clamp V CC Comp. 0.1 V

Pin No. Equivalent circuit Description I/O

3 TON: ¾

Terminal for connecting the capacitor for deter- mining the maximum on-period (high) of the IC output (VOUT ). An equation for approximate calculation of the maximum on-period (TON ) is as follows: TON = 6 500 · C C: External capacitance

4 CLM: I

Input terminal for detection of the pulse-by- pulse overcurrent protection. Normally, it is recommended that a filter be attached externally.

5 GND: ¾

Grounding terminal. 6V OUT :O Output terminal for directly driving the power MOSFET. It uses the totem pole type output. The maximum rating of the output current: Peak: –1 A DC: –150 mA 7V CC : ¾ Terminal for applying power supply voltage. It monitors the supply voltage and has the opera- tion threshold of start/stop/OVP reset. n Terminal Equivalent Circuits (continued) FB Comp. 0.7 V V CC V REF (-) Comp. V REF V CC

Pin No. Equivalent circuit Description I/O

8 TIM/OVP: I

Terminal for use both OVP (overvoltage protec- tion circuit) and timer latch. [OVP] When overvoltage signal of the power supply is detected and high is inputted to the terminal, it turns off the internal circuit. At the same time, it holds that condition (latch). To reset the OVP latch, the V CC should be decreased to a voltage lower than the release voltage. [Timer latch] It detects the output voltage fall due to the overcurrent condition of the power supply out- put through the current level inputted to IFB. When the I IFB decreases under the current of certain value, the charge current flows in the capacitor which is connected to this terminal. Then, when the capacitor is charged up to the threshold voltage of the OVP, the OVP works so that the IC could keep the operation stop condi- tion.

9 FB: I

Terminal for connecting the photocoupler for error voltage feedback of the power supply out- put. It is possible to cancel about 180 mA of the dark current of photocoupler. n Terminal Equivalent Circuits (continued) 5 mA 7.3 VComp. V CC TON 9 V CC n Application Notes [1] Operation descriptions 1. Start/stop circuit block

  • Start mechanism When AC voltage is applied and the supply voltage reaches the start voltage through the current from start resistor, the IC starts opera- tion. Then the power MOSFET driving starts. Thereby, bias is generated in the transformer and the supply voltage is given from the bias coil to the IC. (This is point a in figure 1 ) During the period from the time when the start voltage is reached and the voltage is gen- erated in the bias coil to the time when the IC is provided with a sufficient supply voltage, the supply voltage of the IC is supplied by the capacitor (C8) connected to V CC . Start voltage Stop voltage a b Start condition Start failurec Voltage supplied from bias coil Before start Start V CC V OUT GND After AC rectification Figure 1 Start resistance
  1. Start/stop circuit block (continued)
  • Start mechanism (continued) Since the supply voltage continuously decreases during the above period (area b in figure 1), the power supply is not able to start (state c in figure 1), if the stop voltage of the IC is reached before the sufficient supply voltage is supplied from the bias coil.
  • Function The start/stop circuit block is provided with the function to monitor the VCC voltage, and to start the operation of IC when VCC voltage reaches the start voltage (14.9 V typical), and to stop when it decreases under the stop voltage (8.6 V typical). A large voltage difference is set between start and stop (6.3 V typical), so that it is easier to select the start resistor and the capacitor to be connected to V CC . Note) To start up the IC operation, the startup current which is a pre-start current plus a circuit drive current is necessary. Set the resistance value so as to supply a startup current of 350 mA. 2. Oscillation circuit The oscillation circuit generates the pulse signal for turning on/off the power MOSFET by using charge/ discharge of the C2, R2 and C3 connected to TOFF (pin 2), TON (pin 3) respectively. The concept of constant voltage control at the time of making up the switching power supply is fixing the off- period of the power MOSFET and achieving the control by changing the on-period. This on-period control is performed by directly changing the output pulse width of the oscillation circuit. During the on-period of the power MOSFET, the C2 is charged to the constant voltage (approximately 0.9 V). On the other hand, the C3 is charged from almost 0 V by the charge current from the TON terminal. When the voltage across the both ends of the C3 reaches approximately 0.7 V (T a = 75°C), the oscillation circuit output is reversed and the power MOSFET is turned off. At the same time, the C3 is rapidly discharged by the discharge circuit inside the IC and its voltage across the both ends becomes almost 0 V. The charge current from the TON terminal is changed by the feedback signal to the FB terminal (pin 9). (Described later.)

Figure 2. Oscillation circuit operation oscillation waveform at the time when the TDL terminal is pulled down to the GND.

n Application Notes (continued) [1] Operation descriptions (continued) 7. Overvoltage protection circuit (OVP) (continued)

  • Operating supply current characteristics While the OVP is operating, the decrease of the supply current causes the rise of the supply voltage VCC , and in the worst case, the guaranteed breakdown voltage of the IC (35 V) can be exceeded. In order to prevent the rise of supply voltage, the IC is provided with such characteristics as that the supply current rises in the constant resistance mode. This characteristics ensure that the OVP can not be released unless the AC input is cut, if the supply voltage V CC under OVP operating has been stabilized over the OVP release supply voltage (which depends on start resistor selection). (Refer to figure 9.) 8. Timer latch When the short-circuit or overload of the power supply output continues for a certain period, the pulse-by-pulse overcurrent protection is not sufficient for protection of the transformer, fast recovery diode (FRD), Schottky diode in the secondary side, and the power MOSFET. For this reason, the timer latch function is employed, which stops the power supply by hitting the OVP when the overcurrent condition continues for a certain period. The short-circuit or overload of the power supply output is monitored as the decrease of the power supply output (at this time the pulse-by-pulse overcurrent protector is in the operating condition). The decrease of the power supply output is detected as the decrease of current amount at the current feedback terminal of IFB. When the decrease amount of this current exceeds a certain value, the comparator inside the IC reverses, and the constant current flows to the TIM/OVP terminal. The external capacitor is connected to the TIM/OVP terminal. Electric charges are accumulated in this capaci- tor, rising the OVP terminal voltage. When the OVP operating threshold voltage (7.3 V typical) is reached, the OVP starts operation to stop the IC, and keeps this stop condition. (Refer to figure 10.)
  • Timer period The period from the time when an error of the power supply output is detected to the time when the OVP starts operation (hereinafter referred to as "timer period") should be longer than the rise time of the power supply. Since at operation start the IC is in the same condition as the overload or output short-circuit condition, if the timer period is shorter, the power supply works latch and can not start. Therefore, the IC is designed so that the timer period can be set arbitrarily by the value of the external capacitor connected to the TIM/OVP terminal. However, particular care should be taken, because too large value of this capacitor may cause the breakdown of the power supply. Figure 9 ICC V CCV CC - OVP V CC V OUT GND After AC rectification Start resistor The current supply from the start resistor continues as long as the voltage of the power supply input (AC) is given. After OVP starts operation, since the output is stopped, this bias coil does not supply current. * Select the resistance value so that the following relationship can be kept by current supply from the start resistor: V CC > VCC -OVP At VCC -OVP (voltage under which OVP is released) as the boundary, the operating current is temporarily increased. This prevents V CC from exceeding the breakdown voltage due to the current supplied from the above start resistor.

n Application Notes (continued) [2] Design reference data

  • How to start the soft start function by external parts The power supply rises under overload condition due to the capacitor connected to the power supply output. In this condition, since the voltage of the power supply output is low, the normal constant voltage control attempts to rise the power supply output at the maximum duty. The control uses the pulse-by-pulse overcurrent protection (CLM), attempting to limit the current. However, the pulse can not be brought down to zero due to delay of filter, etc. As a result, a large current flows into the main switch (the power MOSFET) or the diode in the secondary side, and in the worst case these parts are damaged. For this reason, the soft start function is used to suppress the rush current at start of the power supply. As shown in figure 12, the R3 and C4 are connected between the FB terminal (pin 9) and the GND terminal (pin 5) to use the soft start function. When the supply voltage of the IC reaches the start voltage and the start circuit begins operation, the open bias is outputted to the FB terminal. By this voltage, the charge current flows in the C4 to become flowing-out current of the FB terminal (I FB ), and out- put control begins under the condition in which TON period is short. The IFB decreases with elapse of time, since it changes proportionally to the charge current of the C4. The CLM circuit operates, depending on the sum of the R7 end-to-end voltage by the current which flows in the power MOSFET at power-on and the R6 end-to-end voltage by the charge current of the C4. Therefore, since the current which flows in the power MOSFET at power-on gradually increases, the rush current can be suppressed. GND Start resistor V CC FB V OUT After AC rectification Figure 12 R8 C6 CLM To AC (-)

n Application Circuit Example 68 kW 10 kW AC 22 W 82 mF R10 PC1 C10 2 200 pF R11 R12 AN1431T/M FRD 1 800 pF 0.033 mF 1 000 pF 560 mF Out6 CLM4 TIM/ OVP 8 TDL 1 V CC 7 GND5 TON3 FB9 TOFF2 1.5 kW

680 WR2

1.5 kW 15 kW 2 200 pF 1 mF 470 pF SBD PCI R7 0.11 W OVP TDL High-clamp Low-clamp FB CLM VF control U.V .L.O. V REF Drive