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Remotely adjusting PSU voltage

09 Feb 2016  | Chee Hua How

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This design idea shows a way to adjust the output voltage of a power supply, for OVP/UVP testing, margin-testing the load, voltage programmability, or any other reason.

Figure 1: Circuit for bidirectional trimming of PSU output voltage.

The circuit in figure 1 is capable of bidirectional trimming of a power supply's output voltage by sourcing or sinking current into the feedback node. It can operate either manually with switches or digitally via three inputs: S1 (STEP), S2 (RESET), and S3 (U/D).

Every S1 rising edge increases or decreases VO by one step (about 95mV in this design). S3 controls the adjustment direction (up/down), and S2 resets VO to nominal.

One shot U4B ensures:
1. One incremental step per press (debounces S1).
2. Sufficient waiting period for the UUT's protection circuit to react.

The trim section comprising U5 & U6 features a voltage controlled current sink/source (VCCS). U3B & U3C shift up the VCCS's reference point so that:
1. It's centred at the same level as Vref_ps. Thus, in neutral condition (e.g., upon reset), VW = ½Vref1 ≈ Vref2, Itrim≈0 & VO≈VO(nominal).
2. The circuit can use a single supply while still capable of sinking and sourcing current.

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