TOPSwitch-FX series single chip switching power supply application design

TOPSwitch-FX series single-chip power integrated circuits are widely used in various general and special switching power supplies, standby power supplies, and switching power supply modules.

1. 12V, 30W switching power supply with external current limiting

The circuit of a 12V, 30W high-efficiency switching power supply composed of TOP234Y is shown in Figure 1. Its AC input voltage range is AC85~265V, and the power efficiency can reach 80% when fully loaded. The AC voltage u passes through the electromagnetic interference (EMI) filter (C10, L1) and the input rectifier filter (BR, C1) in turn to obtain the DC high voltage UI. UI is connected to the M terminal after being divided by R1 and R2, which can reduce the limit current as UI increases. R1 can provide a voltage feedforward signal, and when UI is high, it can automatically reduce the maximum duty cycle to reduce the output ripple. R2 is the current limit setting resistor, and the set Ilimit≈0.7Ilimit=0.7×1.5A=1.05A is slightly higher than the peak current Ip value at low voltage input. The coefficient is taken as 0.7 here because the maximum continuous output power Pom=45W of TOP234Y under wide input range, while the actual output power P'om=30M, that is, P'om/Pom=30/45=0.67≈0.7. This design method allows the high-frequency transformer to use a smaller core, reduce the power consumption of TOP234Y by increasing the primary inductance Lp, and prevent magnetic saturation. In addition, due to the use of voltage feedforward technology to reduce Dmax, the switching power supply can work normally even if the input voltage UI and the primary induction voltage UOR are high. It allows the use of low-cost R, C, VD type drain clamp circuit (R3, C7, VD1) to replace the more expensive TVS (transient voltage suppressor) and VD type clamp circuit to absorb the spike voltage generated by the leakage inductance of the high-frequency transformer when TOP234Y is turned off, and protect the drain.

After the secondary voltage is rectified and filtered by VD2, C2, C3, L2 and C4, a regulated output of +12V and 2.5A is obtained. To reduce the loss of the rectifier tube, VD2 uses the MBR1060 10A/60V Schottky diode. C9 and R7 are connected in parallel at both ends of VD2 to prevent VD2 from self-oscillating (ringing) under high-frequency switching conditions. When the switching power supply is unloaded, TOPSwitch-FX can further reduce the maximum output duty cycle by skipping cycles, making Dmax <1.5%. Therefore, there is no need to connect a dummy load at the output end, which can also reduce the power consumption in no-load or standby states.

The power supply uses an optocoupler feedback circuit with a voltage regulator. IC2 is an LTV817A linear optocoupler. VDZ uses a 1N5240C voltage regulator, and its stable voltage Uz=10 (1±0.02) V. The forward voltage drop UF≈1V of the LED in the optocoupler. The output voltage is determined by the following formula:

Uo=Uz+UF+UR4

The voltage stabilization principle is analyzed as follows: When Uo↑ due to some reason, Uo>U2+UF+UR4, the error voltage Ur'=Uo-(Uz+UF+UR4) generated will make the IF of LED↑, and after the optical coupler, the IE of the receiving tube↑, so that the control end current Ic↑, and the duty cycle D↓, resulting in Uo↓, thus achieving the purpose of voltage stabilization. Conversely, Uo↓→IF↓→IE↓→Ic↓→D↑→Uo↑, which also plays a role in voltage stabilization.

The typical value of the stable current of 1N5240C is 20mA. When R4=150Ω, it can only supply 6.7mA of current. Further increasing the resistance value will be limited by the LED working current IF (usually 3.5~7mA). For this reason, the resistor R6 provides another 13.3mA working current, so that the stable current Iz of VDz=3.7mA+13.3mA=20mA, and its voltage regulation characteristics are also improved. After the feedback winding voltage is rectified and filtered by VD3 and C6, a 12V feedback voltage is generated, which is passed through IC2 to provide bias voltage to the control end of TOP234Y. C5 is a bypass capacitor, which also forms a compensation circuit of the control loop with R5.

2. 35W set-top box switch circuit with multiple outputs

The 35W set-top box switching power supply circuit with 5 outputs is shown in Figure 2. The 5 voltages are: Uo1 (+30V, 100mA), Uo2 (+18V, 550mA), Uo3 (+5V, 2.5A), Uo4 (+3.3V, 3A), Uo5 (-5V, 100mA). Among them, +5V and +3.3V are the main outputs, and the rest are auxiliary outputs. When the AC input voltage u=220(1±0.15)V, the total output power reaches 38.5W; if a wide range voltage input (u=85～265V AC) is used, the total output power is reduced to 25W, which can be used as a switching power supply in set-top boxes (Set-top Box), video recorders (VCR), video camcorders (CVCR) and DVDs. The power supply uses three ICs: TOP233Y (IC1), optocoupler LTV817A (IC2), and adjustable precision shunt regulator TL431C (IC3). In order to reduce the volume of the high-frequency transformer and enhance the degree of magnetic field coupling, the secondary winding adopts a stacked winding method. The absorption loop composed of R4 and C14 can reduce the interference of radio frequency noise on video equipment such as televisions. If necessary, the switching frequency selection terminal (F) can be connected to the control terminal (C) to select the half-frequency mode to further reduce the sensitivity of the TV to video noise.

R6, R7 and R8 are proportional feedback resistors, which make the 5V and 3.3V power supplies feedback in a certain proportion. The load regulation rate of these two outputs can reach ±5%. R9 and C16 constitute the frequency compensation network of TL431C. C17 is a soft start capacitor. When C17=22 μF , the soft start time can be increased by 4ms. Adding the original 10ms soft start time, the total is 14ms. The other outputs are not fed back, and the output voltage is determined by the turns ratio of the high-frequency transformer. Because the output power of the -5V power supply is very low, the voltage is regulated by resistor R2 and voltage regulator VDz2. R9 is a dummy load of the +30V output, which can reduce the no-load and light-load voltage of the circuit. In view of the large output power of the 5V, 3.3V and 18V power supplies, the three have added post-stage LC filters (L3 and C9, L4 and C11, L2 and C7) to reduce the output ripple voltage.

TOP233Y has a frequency jitter characteristic, which is very helpful in reducing electromagnetic interference. In addition, by reasonably selecting the component values ​​of the safety capacitor C15 and the EMI filter (C6, L1), the electromagnetic radiation generated by the switching power supply can meet the CISPR22 (FCCB) international standard. Connecting one end of C15 to the positive pole of U1 can minimize the common-mode interference of TOP233Y. It should be pointed out that C15 and C6 are both called safety capacitors. The only difference is that C15 is connected between the high voltage and the ground, which can filter out the common-mode interference generated by the primary and secondary coupling capacitors. It is called "Y capacitor" in the IEC950 international standard. C6 is connected to the AC power supply input line end, which is specifically used to filter the series mode interference between the power grid lines and is called "X capacitor".

In order to withstand the lightning voltage that may come from the power grid, a varistor VSR with a nominal voltage of U1mA=275V is connected in parallel at the AC input. U1mA represents the voltage value across the varistor when a 1mA DC current passes through it. 3. 17W PC standby power supply with 5V and 3.3V output

The PC standby power supply circuit that can provide two main outputs of 5V, 2A and 3.3V, 2A is shown in Figure 3. The power supply also adds an auxiliary output of 15V, 30mA at the lowest cost. A TOP232Y single-chip switching power supply is used in the circuit, and the total output power is 17.05W. The DC input voltage range is 200-375V. A 220/110V AC fixed input voltage can also be selected, and only a rectifier filter is connected, without adding an input voltage doubler to double the 110V AC. This design gives full play to the excellent characteristics of TOP232Y such as soft start, undervoltage protection, strict current limiting characteristics and high switching frequency, so that the high-frequency transformer can select EE19 type magnetic core. In addition, since TOP232Y increases the distance between the high-voltage drain terminal and the low-voltage terminal, the leakage between the pins is reduced, so the power supply can be used in a harsh environment. C1 is a high-frequency decoupling capacitor for DC high voltage, and C1 can be omitted when U1 is very close to the standby power supply. The line detection circuit R1 is used to set the undervoltage value UUV. When R1=3.9MΩ , UUV=IUV·R1=50 μA×3.9MΩ=UI>195V DC , the power supply is reconnected.

The feedback winding voltage is rectified and filtered by VD4 and C6 to generate a 15V feedback voltage, which is used as a +15V output (not isolated from the primary) on the one hand, and provides bias to the control end of TOP232Y through a phototransistor on the other hand. R4, R6 and R7 are sampling resistors used to detect the change in the output voltage of 3.3V and 5V. R2 is the limiter of the LED TL431C provides bias current. C8 is a soft-start capacitor that can eliminate the voltage overshoot phenomenon generated when the power is just turned on. The feature of TOP232Y skipping cycles when unloaded can meet the low power consumption index of the PC standby power supply.