Below are basic guidelines to select the component values for the injection network:

1. The undamped loop gain characteristic is likely to be unstable within the range of 5 kHz to 50 kHz, which,

as mentioned previously, relates to an LC resonance between CM choke inductance and inject capacitance.

Observe from circuit simulation – or by using the TPSF12C3 quickstart calculator – the frequency, fLFstability,

at which the phase crosses –180° with positive gain, indicating negative gain margin.

2. Choose a corner frequency with RD1 and CD2 equal to one fifth of the instability frequency:

(2)

Assigning RD1 = 1 kΩ and assuming instablity at 35 kHz, use Equation 3 to find a value for the capacitance

of CD2:

(3)

3. Select CD1 < CD2, where a typical choice is CD1 = CD2/5 = 4.7 nF.

4. Choose the resistance of RD2 such that the RD2, CD2 corner frequency is equal to that of RD1, CD1:

(4)

5. Select the resistance of RD3 to damp the resonance around the instability frequency, fLFstability.

• A typical choice for RD3 is 500 Ω to 1 kΩ.

• Assign CD3 equal to CINJ or a suitable value such that the RD3, CD3 corner frequency is less than

switching frequency.

• A lower resistance for RD3 results in more damping but at the penalty of reduced high-frequency

attenuation (or forces a higher value for CD3 to maintain the applicable corner frequency below the

switching frequency).

6. Select a resistance for RD1A of 50 Ω to improve the phase margin of the AEF loop (if needed).

9.2.1.2.5 Surge Protection

While the sense pins have internal clamp protection, the higher value of inject capacitance produces larger

currents during surge events and thus requires external protection. Place a bidirectional TVS diode on the

low-voltage side of the inject capacitor with standoff voltage of 24 V. Using the SOD-323 packaged device given

in Table 9-2, clamping occurs at 40 V and 50 V with surge currents of 1 A and 8 A, respectively.

TPSF12C3

SNVSCB8 – MARCH 2023

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