# Push Pull Switch Mode Power Supplies

I am reading this material for understanding the switch mode push pull power supplies. Push Pull Switched Mode Power Supplies

I am unable to understand the working though. Can someone provide a simple explanation for the circuit in 3.4.2 and its waveforms?

And it is mentioned as R3 and C2 are the timings components which I dont understand also. How do the internal transistors and the external transistors function? Please explain. Thanks.

• If you haven't already, strongly recommend reading modules 1-3.4.1 – K H Jan 16 at 4:39

The working of the circuit is described here in this document which describes the functionality of PWM modulators.

RT and Ct are part of an oscillator circuit where

$$\ f = \frac{1.18}{R_T C_T}\$$

(Rt is in kΩ, Ct is in uF, f is in kHz)

Practical values of CT fall between 1 nF and 100 nF. Practical values of RT fall between 1.8 kW and 100 kW. This results in a frequency range typically from 120 Hz to 500 kHz.

Source: datasheet listed above

• Thanks. Can you provide a run through of the operation of the blocks during one on state and one off state? – Electronic_Maniac Jan 16 at 5:09
• And sorry for the basic question, how does the capacitor charge and discharge through the resistor. From where the R and C get the charge and discharge? – Electronic_Maniac Jan 16 at 5:17
• Is this homework? – laptop2d Jan 16 at 5:55
• No I am trying to understand the working of the circuit – Electronic_Maniac Jan 16 at 6:03
• I am unable to understand how the RC section works. We will get a ramp waveform at the timing capacitor pin only when RC is connected in series, right? But here R and C are not in series. How is this ramp generated? – Electronic_Maniac Jan 16 at 6:45