FLYBACK CALCULATIONS - PART VII

In this section we will discuss the snubber network for the transistor switch on the primary side of the converter (see Fig.1. Click on the figure to see a larger view.) First an estimate is made for the voltage spike that is to be snubbed as 2.5 times the reflected voltage from the secondary as

Vsn = 2.5 * VRO

With VRO previously determined to be 350 volts (see previous posts), we have

Vsn = 875 volts

Next we use this estimate to calculate an approximate value for the power dissipated in the snubber network.

Psn = 0.5 * Fs * Llk * Idspeak^2 * Vsn / (Vsn -VRO)

where Fs is the switching frequency, Llk is the transformer leakage inductance, and Idspeak is the peak transistor switch current. Our calculation results in

Psn = 0.316 watts

where the data we used is from our design example as

Fs = 100KHZ
Llk = 141 microhenry
Idspeak = 0.164

Now we can calculate a rough estimate of what the snubber resistor value should be (usually it is necessary to change the value based on actual snubber network performance but it gives us a starting point.)

Rsn = Vsn^2 / Psn = 2.4 megohm

(In the actual design, this resistor was changed to 360K.)

Using the 2.4 megohm value for the snubber resistor we will calculate the ripple voltage on the capacitor we have


dVsn = Vsn / ( Csn * Rsn * Fs ) = 3.6 volts

with Csn set to 0.001 microfarad. Note that if the value of Csn is set too hihg the operation of the flyback converter will not work well, e.g., values above 0.01 microfarad usually do not provide good results depending on the other values used in the design such as the primary inductance value. Remember that the primary inductance and the snubber capacitor form a "tank" circuit that can resonate and this is not usually desirable in a standard flyback converter.

We now have a preliminary design for the snubber network.