CCFL push-pull buffer circuit

Drain voltage without suppression

Figure 1 details the typical gate drive voltage and drain voltage waveforms for a push-pull driver operating from a 15V DC supply . In a push-pull driver configuration, when the complementary MOSFET turns on, the drain voltage normally rises to twice the DC supply voltage (or 30V in this case). However, as shown in Figure 1, the voltage spike is as high as 54V. The drain of the n-channel power MOSFET also sees a voltage spike when the MOSFET turns off and when the complementary MOSFET turns on.


Figure 1. Drain voltage without a buffer circuit

Circuit and design for suppressing drain spike voltage

The voltage spikes can be suppressed by adding a simple RC network to each drain, as shown in Figure 2. The appropriate resistor (R) and capacitor (C) values ​​can be determined by the following process. After explaining the process, an example will be given to demonstrate how to reduce the voltage spikes shown in Figure 1.


Figure 2. Drain buffer circuit for push-pull driver

Determine the appropriate snubber circuit RC value:

1. Measure the peak resonant frequency. See Figure 3 for an example.
2. Connect a capacitor (no resistor, only capacitor) in parallel to the drain and source of the MOSFET, and adjust the capacitance value until the peak resonance frequency is reduced to half of the original value. At this point, the capacitance value is three times the parasitic capacitance value that generates the peak voltage.
3. Since the parasitic capacitance value is known, the parasitic inductance value can be obtained using the following equation:
   L = 1 / [(2πF)² x C], where F = resonant frequency and C = parasitic capacitance
4. Now that the parasitic capacitance and inductance values ​​are known, the characteristic impedance of the resonant tank can be found as follows:
   Z = SQRT(L/C), where L = parasitic inductance and C = parasitic capacitance
5. The resistor value in the RC snubber circuit should be close to the characteristic impedance, and the capacitor value should be four to ten times the parasitic capacitance value. Using a larger capacitor can slightly reduce the voltage overshoot, but at the expense of more power dissipation and lower inverter efficiency.

Calculating RC Snubber Component Values

In this section, using the five steps mentioned above, you can calculate the appropriate resistor and capacitor values ​​that form the snubber circuit to reduce the spike voltage in Figure 1.

1. Find the frequency of the resonant spike voltage. Figure 3 shows that it is about 35MHz.
   
   
   Figure 3. Frequency of the resonant spike voltage without a snubber circuit
   
   
2. A capacitor is added between the drain and ground to reduce the resonant frequency to about half (17.5MHz). As shown in Figure 4, a 330pF shunt capacitor reduces the resonant frequency to about 17.5MHz. The optimal capacitor value can be determined by trying different capacitor values ​​in parallel. It is best to start with a small capacitor value (such as 100pF) and then gradually increase it.
   
   Because a 330pF shunt capacitor reduces the resonant frequency to half, the parasitic capacitance value should be one-third of that (about 110pF).
   
   
   Figure 4. Resonant spike voltage frequency when a 330pF shunt capacitor is provided
   
   
3. Calculate the parasitic inductance value.
   Parasitic inductance = L = 1 / [(2 x 3.14 x 35MHz)² * 110pF] = 0.188µH
   
   
4. Calculate the characteristic impedance.
   Characteristic Impedance = Z = SQRT (0.188µH / 110pF) = 41
   
   
5. Choose appropriate resistor and capacitor values. The resistor value R in the snubber circuit should be close to 41Ω, while the capacitor value C should be between four and ten times the parasitic capacitance of 110pF. In this example, we choose capacitor C to be 1000pF, which is about nine times the parasitic capacitance value.
   
   Figure 5 shows the result after adding a snubber circuit consisting of a 39Ω resistor and a 1000pF capacitor.
   
   
   Figure 5. Drain voltage after adding an RC snubber circuit (39Ω, 1000pF)
   
   

in conclusion

This application note shows that by some simple empirical measurements, the appropriate values ​​for the RC snubber circuit in a push-pull drive configuration can be determined. This snubber circuit can significantly reduce the unwanted voltage spike that appears at the drain of the power MOSFET.