Power Supply current limit speed

[Chris Penrose]

I am designing a low cost production test system around the BeagleBone.
I am hoping the Hydra triple power supply will be good fit to supply the lower voltages.
Our products are generally 24VDC powered, so I will still need to design a current limited 24V supply.

My concern is the following statement in the Datasheet:
‘It is important to note that this feature is a slowly-responding software feedback loop, and is not sufficient to
prevent damage to circuits that require tight control of current and/or supply voltage.’

If I configure the Hydra to supply 5VDC at 100mA, will it destroy any faulty product before current limiting kicks in?

[CalebModerator]

Hi Chris,

It could, yes. Also note that the Hydra can’t handle input voltages of 24 V, so you’d need to step the input down to get the voltage that you want.

[Chris Penrose]

The 12V supply
I would be via a 24/12VDC buck converter.
Similar to the Pololu S18V20F12 (12V / 2A out) but probably bigger.

Destroying faulty product
Could you clarify ‘It could, yes’ with some data.
For example:
I set 5VDC with a 100mA current limit, output disabled.
What is the transient voltage and current response when:
- Enabled into a 220uF capacitor in parallel with a 47 ohm resistor (i.e. driving a typical power supply / device).
- Enabled into a 220uF capacitor in parallel with a 1 ohm resistor (i.e. driving a broken (shorted) power supply).
I am hoping that the voltage will remain below say 5.5V and the peak current remains below 150mA.

[chrwebKeymaster]

Hi Chris,

We haven’t characterized the transient performance under those particular conditions. However, note that the Hydra only switches to current-control mode if the maximum current is surpassed - meaning, if you set the supply at 5V and 100mA max current, the supply voltage won’t go above 5V, and current regulation only turns on to drop the voltage if 100mA is exceeded at 5V.

[Chris Penrose]

Hi,

I can not commit to designing your power supply into our test system without being confident the current limit transient performance will protect the ‘device under test’. Specifically I need to know this at 5V / 100mA.
Initially I will be building three test systems, and more as required.

The ‘Hydra Datasheet’ from your web site is the only detailed specification I am aware of.
This contains constant voltage mode transient responses as follows:
- 60mA / 2100mA showing that 5V spikes down to 4.5V.
- 2100mA / 60mA showing that 5V spikes up to 5.5 then down to 4.5V.
These transients are not great, but are not a show stopper.

Without the 5V / 100mA current limit transients I can not be confident the ‘device under test’ will not be damaged.
This is to show stopper.
I am sorry if I am asking for something that is not easy to produce, but I am fairly sure others will also appreciate this information.

[CalebModerator]

The transients shown in the datasheet do do not involve the software current-limit, but rather represent the transient response of the switching regulators on their own. The transient performance for current-control will be worse.

“Without the 5V / 100mA current limit transients I can not be confident the ‘device under test’ will not be damaged.
This is to show stopper.
I am sorry if I am asking for something that is not easy to produce, but I am fairly sure others will also appreciate this information.”

You should not rely on the Hydra’s software current-control loop to protect your electronics in the event of an onboard short. If you need that functionality, I’d recommend using a traditional bench-top supply with adjustable current limit.

• This reply was modified 1 year ago by  Caleb.

[Chris Penrose]

Thanks for your honesty.
That is a shame as the other features of the hydra power supply were a good fit.
I will continue designing an appropriate power supply.

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