Optimizing Adapter Power Supply Design

Jul 1, 2010 12:00 PM
Jason Sun, Fairchild Semiconductor, China

PERFORMANCE TESTING

As shown in Figure 7, the average efficiency over the whole voltage range is more than 87%. Regulation rate as shown in Figure 7, the variation between output voltage and input voltage is within 2%. As shown in Figure 8, the PF stands above 0.9 over the whole input voltage range, and the THD is less than 15%, delivering a very good PFC. As shown in Figure 9, the no-load standby power consumption is compliant with Energy Star specifications (50>W>250W, Pin0.5W). As shown in Figure 10, the maximum operating voltages and currents of the primary side MOSFET and secondary side rectifier are all within the specified range.

When setting up the power supply, special attention should be paid to the settings of several pins on the FAN6921, as well as the switch timing control of the synchronous rectified signal. In other words, we have to carefully adjust some settings.

More specifically, apply a proper voltage on PIN3 (INV) through the divider circuit formed by R23, R27, R39 (Figure 6) to hold the minimum output voltage (Vo_minPFC) at 240V. Also, set the maximum output voltage (Vo_maxPFC) to 400V through Pin1 (Range). This could be implemented because when the input voltage rises, the voltage on Pin13 (VIN) also increases; when VIN rises beyond 2.1V, it drags down the voltage on Range, causing the PFC output voltage to rise due to R34 and R39 being in parallel. If the design only requires the 400V output, remove the R34 on the Range and change the value of the remaining R39 to the parallel resistance of the original R34 and R39. The two voltages can be calculated as:

(See eq. 38)

(See eq. 39)

PFC BROWN-OUT TUNING

When the input voltage drops, the voltage on Pin13 (VIN) also drops; when the voltage of the VIN falls below 1V, the FAN6921 will force the VDD to work in hiccup mode. In this case, both the PFC and the PWM will disable until the VDD voltage is re-established and VIN is higher than 1V. This is how the brown out function works. When setting up the design for the first time, remove the PWM MOSFET (Q2), work only on the PFC element of the design and then plug Q2 to set up the PWM element.

Pin10 (DET) consideration

It has three functions:

MOSFET valley voltage detection, which controls low voltage conduction, namely it detects whether DET voltage is below 0.7V. If so, conduction is enabled.

Overvoltage protection, which locks the IC once DET voltage is higher than 2.5V. The locked IC can only restart after it is powered off. It is recommended that the normal DET voltage is set to around 2V.

Overpower compensation, which detects the current (IDET) going through R15. When the input voltage rises, IDET also increases, and this drags down the overpower limit voltage. So, by regulating the bias resistance (R15) on the DET, the overpower protect point can be held unchanged when the PWM input voltage is between 240V and 400V.

OVERTEMPERATURE PROTECTION

Pin12 (RT) implements overheat protection in two ways:

By using a thermal resistor as a temperature sensor. In this case, the controller is locked only after the RT voltage remains below 0.8V for a time greater than 10 ms.

By using an external component (for example, an optically coupled circuit) to control Pin12. In this case, the controller will be immediately locked once the RT voltage drops below 0.5V for 100µs.

When setting up the DC/DC, it is better to replace the synchronous rectifier controller with a Schottky diode. This avoids damage to the DC/DC controller and the MOSFET that can be the result of improper settings. Set the synchronous rectifier after the PWM has been set up. In reference to Figure 11, disconnect Pin1 (DET) and apply a 5V DC voltage on the pin. We do this to check whether the synchronous rectifier is working normally. If the rectifier is abnormal, regulate the values of R1, R2, R3 and R4, with reference to the DET and MOSFET waveforms, to ensure that the synchronous MOSFET turns on at the same time as the primary MOSFET and turns off about 2µs ahead of it.

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