# Different type of load impact on power stage transfer function

Ming Sun / December 06, 2022

9 min read • ––– views

## Resistive load

The Buck converter test bench in Simplis is as shown in **Fig. 1**. In **Fig. 1** model, we are using resistive load at Buck converter output.

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The **G _{vd}** simulation from Simplis is as shown in

**Fig. 2**.

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## DC current source as the load

In **Fig. 3** model, we are using ideal DC current source at Buck converter output.

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The **G _{vd}** simulation from Simplis is as shown in

**Fig. 4**.

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## Active load with feedback control

Finally, let us use an active load at Buck converter's output. Here the **vcvs** is to mimic an ideal Opamp. The **vccs** and the **100MΩ** is to mimic an **NMOS** device.

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The **G _{vd}** simulation from Simplis is as shown in

**Fig. 6**.

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## Comparison

Next, we can export the data from Simplis as a **csv** file and plot the comparison by Matlab as shown in **Fig. 7**.

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In **Ref. [6]**, the **G _{vd}** transfer function is given by:

The complexed pole frequency is given by:

From **Eq. 2**, the complexed pole frequency is determined by the inductor value **L** and capacitor value **C**. Therefore, the type of load at Buck converter output does not impact the complexed pole frequency.

The quality factor **Q** impact the peaking at complexed pole frequency. The quality factor **Q** is given by:

From **Eq. 3**, we can see the loading impedance has a direct impact on the quality factor **Q**. In the ideal current source and active load case, ideally no matter how the **V _{OUT}** changes, the current should stay the same. Therefore, the small signal output impedance is inifinity. As a result, both the ideal current source and active load shows much higher peaking than the resistive load.

## Conclusion

The loading impedance does not impact the complexed pole frequency. However, it has a direct impeact on the quality factor Q. Ideal current source and active load shows a higher quality factor Q at the complexed pole frequency.

## References and downloads

[1] Fundamentals of power electronics - Chapter 2

[2] Open-loop Buck converter with active load in Simplis - download

[3] Open-loop Buck converter with active load in Simplis - pdf

[4] Open-loop Buck converter with resistive load in Simplis - download

[5] Open-loop Buck converter with DC current source as the load in Simplis - download

[6] Power stage transfer function cheatsheet - Voltage Mode

[6] Power stage transfer function cheatsheet - Voltage Mode (pdf)