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ZHCSJK0A April 2019 – September 2019 UC1843B-SP

PRODUCTION DATA.

8.2.2.3 RCD Diode Clamp

For the UC1843B-SP design a resistor and capacitor are used. The resistor and capacitor is generally a value that is found through testing, but starting values can be obtained. To figure out the resistor and capacitor needed for the RCD clamp, one must first pick how much the node is allowed to overshoot. The equation for finding the voltage of the clamp is Equation 17.

Equation 17.

V_{c l a m p} = K_{c l a m p} \times N_{p s} \times (V_{o u t} + V_{D i o d e})

Note that K_clamp is recommended to be 1.5 as this will allow for only around 50% overshoot. Knowing the parasitic inductance of the transformer and how much the snubber voltage is allowed to change over the switching cycle, can allow one to figuring out starting values for the resistor and capacitor using Equation 18 and Equation 19.

Equation 18.

R_{c l a m p} = \frac{{V_{c l a m p}}^{2}}{\frac{1}{2} \times L_{l e a k a g e} \times {I_{P r i P e a k}}^{2} \times \frac{V_{c l a m p}}{V_{c l a m p} - N_{p s} \times (V_{o u t} + V_{D i o d e})} \times f_{o s c}}

Equation 19.

C_{c l a m p} = \frac{V_{c l a m p}}{Δ V_{c l a m p} \times V_{c l a m p} \times R_{c l a m p} \times f_{o s c}}

A starting value of 10% is generally used for ΔV_clamp.