SLLA566A September 2021 – October 2021 SN6501 , SN6501-Q1 , SN6505A , SN6505A-Q1 , SN6505B , SN6505B-Q1 , SN6505D-Q1 , SN6507-Q1

2 Analysis of Emissions in Push Pull Topologies

To avoid ringing during the dead time, snubbers can be added to the switching nets to dampen them out. The concept of snubbers is simple – the effective impedance is designed such that it acts as transparent medium for the switching frequency (to avoid any signal degradation in driving nets), and act as a resistor for the ringing frequency (to dampen the oscillations and hence the emissions at that frequency). The snubber circuit is implemented as series capacitor and resistor.

Leakage inductances of a transformer can be measured practically using an LCR meter by probing the winding ends and shorting the secondary side of the transformer. The measured impedance (L_PAR) at an appropriate frequency where X_leakage dominates R_winding will yield the leakage inductance of the transformer. To estimate the parasitic capacitance, the gate of the driver needs to be biased appropriately and an impedance analyzer can be used to measure the capacitance (C_PAR) on both D1 and D2 nets on the PCB.

Ringing frequency can be calculated as:

Equation 1.

f_{r e s o n a n c e} = \frac{1}{2 π \times \sqrt{L_{P A R} \times C_{P A R}}}

With the snubber in place - at the ringing frequency, the Q of the circuit can be defined as:

Equation 2.

Q = \frac{\sqrt{\frac{L_{P A R}}{C_{P A R}}}}{R_{s n u b b e r}}

Designing for a Q of 1, yields damped oscillations:

Equation 3.

R_{s n u b b e r} = \sqrt{\frac{L_{P A R}}{C_{P A R}}}

The corner frequency for the snubber can be made equal to the ringing frequency:

Equation 4.

Z_{s n u b b e r} = R_{s n u b b e r} + \frac{1}{j ω \times C_{s n u b b e r}}

Equation 5.

C_{s n u b b e r} = \frac{1}{2 π \times R_{s n u b b e r} \times f_{r i n g i n g}}