Designing power supply

[VSef]

Hello,

I need to design a power supply that is connected to the utility (220V rms) and outputs both 22Vdc and 5Vdc. I have a 220V to 24V transformer after which I have a full bridge rectifier and a capacitor to maintain proper DC voltage of about 33Vdc. I have loads that consume 100mA from my 22Vdc supply and about 80mA from my 5Vdc supply.
I have several thoughts of how to accomplish this:

#1) Use a buck converter to reduce the 33Vdc to 22Vdc and supply the needed loads. Connected the Vin of a second buck converter (the one that outputs 5Vdc) to the output of the first buck converter.
Would the input current ripple of the 2nd buck converter affect the system? Would the output ripple generated from the 1st buck converter affect the 2nd buck's operation?

#2) Use a buck converter (#1) to reduce the 33Vdc to 22Vdc and supply the needed loads and use a second buck converter (#2) to reduce the 33Vdc to 5Vdc and supply the needed loads. The outs and ins of the buck converters are not connected in this configuration. However if buck #1 fails buck #2 would still operate, where as in configuration #1 the failure of the 1st buck prevents buck #2 from working which is better suited for my application.

How do you think this case should be handled? Do you have any other options?

Thank you

 

[KlausST]

Hi,

Use #2)
Use "power_good" detectors on both supplies (22V and 5V). Maybe additionally on 33V to detect early power fail.

Sometimes you need power sequencing. Then the power_good of the first (in time) supply enables the second supply.
Sometimes there is a problem when the 22V supply voltage is below the 5V supply voltage (22V supply failure). Then use a (schottky) diode from 5V (anode) to 22V (cathode).

Klaus

 

[VSef]

Hi,
Use "power_good" detectors on both supplies (22V and 5V). Maybe additionally on 33V to detect early power fail.

Thank you for your answer. What is the best way to detect an early power fail for the 33Vdc? Are you suggesting connecting the power failure circuitry to the shutdown pin of the buck converters?

Sometimes there is a problem when the 22V supply voltage is below the 5V supply voltage (22V supply failure). Then use a (schottky) diode from 5V (anode) to 22V (cathode).

Can you please elaborate more please? What is the role of the diode here?

Thank you

 

[KlausST]

Hi,

What is the best way to detect an early power fail for the 33Vdc?

Measure the 33V at full load.
Then take 90% of this voltage (maybe less on unstable mains grid) and take this is early_power_fail threshold.
This signal often is used to tell an MCU (via IRQ) to expect a power loss soon and safe all necessary data before power off.

Are you suggesting connecting the power failure circuitry to the shutdown pin of the buck converters?

I don´t think this is usefull. Why swithing power OFF erlier than necessary.
But you may use it for power sequencing or soft start reasons.

Can you please elaborate more please?

If your application doesn´t state this in the datasheet. --> don´t care about it.

What is the role of the diode here?

It simply prevents the 22V_supply_voltage to drop below (5V_supply_voltage - 0.7V)

Klaus

 

[VSef]

Hi,


Measure the 33V at full load.
Then take 90% of this voltage (maybe less on unstable mains grid) and take this is early_power_fail threshold.
This signal often is used to tell an MCU (via IRQ) to expect a power loss soon and safe all necessary data before power off.


I don´t think this is usefull. Why swithing power OFF erlier than necessary.
But you may use it for power sequencing or soft start reasons.


If your application doesn´t state this in the datasheet. --> don´t care about it.


It simply prevents the 22V_supply_voltage to drop below (5V_supply_voltage - 0.7V)

Klaus

Thank you for your reply.

As I was rethinking about my question, another thought came into my mind. Since the utility voltage is not fixed, the DC bus will vary between 30Vdc to 50Vdc. For the buck #1 where I want to generate 24Vdc that is not a problem. However for the 5Vdc don't you think going from 50Vdc to 5Vdc is going to cause a problem? Isn't option #2 better? I was thinking of using LM5018 for this application.

What do you think? And what do you think are the disadvantages of #1?

Thank you