To minimize the electromagnetic interference (EMI) and video noise caused by different signal frequencies inside the same piece of equipment, it's sometimes necessary to synchronize the switching frequency of the power supply (such as for tuner compatibility).
Generally, using a fixed-frequency controller with an external oscillator, or at least an adjustable frequency, solves this. But instead of designing a different specific— and often expensive—dedicated controller for each application, it's simpler and easier to synchronize a variable-frequency controller.
Enter the so-called quasi-resonant, or valley, power supply, which varies switching frequency when the input voltage changes. With this topology, you can use a dedicated controller pin to detect the transformer-core reset or the valley of the switching MOSFET drain voltage. In turn, you can produce a synchronization signal that mimics the expected signal and forces the controller to switch at a constant frequency.
Let's use the NCP1377 quasi-resonant controller (from ON Semiconductor) as an example. However, the method applies to most other existing controllers with a little adaptation. The demagnetization (Demag) pin dictates when the NCP1377 turns ON. As long as the voltage on this pin is above 65 mV, the device's output driver (Drv) stays LOW. When the Demag pin voltage falls below the 65-mV threshold, the driver goes HIGH (Fig. 1). An internal blanking delay (TBLANK) always ensures a minimum duration for TOFF, which prevents the switching frequency from going too high.
As a result, it's possible to use the Demag pin to synchronize turning the controller ON with an external signal. This signal must be HIGH to maintain the OFF state and then go LOW to authorize the turn ON. It must fulfill three major requirements:
- The LOW-state duration must be shorter than the typical 8-µs value of TBLANK to avoid working in fixed TOFF mode. This will occur if the LOW-state duration is longer than TON + TBLANK.
- The LOW-state level must always be below 65 mV, even in a noisy environment. In fact, it would be safer if this level were slightly negative.
- The HIGH-state level must be lower than the typical 7.2-V internal overvoltage reference (VREF) threshold, because the Demag pin also performs overvoltage detection.
Based on these parameters, we can determine the best synchronization signal to apply (Fig. 2). However, it's not practical to generate a negative voltage. So the simplest solution is to use a positive signal and then insert a small circuit (Fig. 3) in series with the Demag pin to shift that signal.
This simple circuit (capacitor C0 in parallel with resistor R0) uses a series resistor (R1) to permit reducing the HIGHstate voltage if needed. R1 divides the voltage applied to the Demag pin thanks to the controller's internal 30-kΩ impedance at that pin.
With this arrangement, the synchronization signal can have a LOW state above 0 V and a HIGH state above 7.2 V. The signal applied to the Demag pin will be shifted down, ensuring a slightly negative LOW state, and the HIGH-state value can be adjusted by changing the value of R1. This resistor isn't needed if the maximum voltage of the Sync signal is less than 5 V.
Figure 4 shows the final Sync signal applied to the Demag pin for two different load conditions (in a, it's at 40% of full load, and in b, it's at full load). In each case, the Drain waveforms depicted demonstrate that the power supply is running in synchronized mode.
However odd it may seem to use a variable-frequency controller to build a synchronized power supply, it does the job perfectly and offers a neat solution when it's not possible or too expensive to use a dedicated controller.