PI design for mains-powered GPRS modem
I am looking for advice on suitable chips/designs for a new project. It is a mains-powered (240V) GPRS modem. GPRS modem modules (as in mobile phones) typically assume a battery and the mains PSU would be mainly there to charge the battery. But the battery is not required in a mains-powered system so this places a different requirement on the power supply, since when the GPRS device transmits there are short (typ. 1ms, with a 20% duty cycle) high current (typ. 2A) current pulses.
My prototype is using an encapsulated ac/dc regulator with large tantalum capacitors on the 5V output, feeding a switch mode buck convertor delivering 3.8V to the GPRS modem. This works fine, but I'd like a cheaper solution.
The GPRS modem can cope with up to 400mV drop during these current pulses.
Suggestions?
Comments
GPRS PSU specs are:
Vout = 3.8V nominal, but may be in the range 4.5V - 3.3V
Vdrop (drop during transmit burst) = 400mV max
Vripple (<200kHz) = 50mV max
Vripple (>200kHz) = 2mV max
Iout (not transmitting) = between c. 5uA and 30mA - for other electronics
Iout (pk, transmitting) = 2A
This Fairchild app note describes the problem space: http://www.fairchildsemi.com/an/AN/AN-8019.pdf
The Fairchild design assumes a current-limited USB power source which is followed by a buck converter with 3000uF of output capacitance. I'm hoping we could find a PI chip which could be used, again presumably with a good deal of output capacitance.
I'd like an isolated design. Present requirements are for 240V ac input, but a design that can take 110V would be a bonus.
You mentioned that this design would also be used to charge a battery. This typically indicates that you'd need both CV (constant voltage) and a CC (constant current) operating area.
I think our LinkSwitch-II devices are going to be a good place for you to start looking: http://www.powerint.com/en/products/linkswitch-family/linkswitch-ii
I would also download and install our PI Expert Software. This will allow you to put in your power supply specs (input voltage and freq, output voltage, output current, etc), design your transformer, etc.
When using a LinkSwitch-II device, you output current for the design is going to be dictated by the charging requirements of your particular battery. This output current will probably be less than your 2A peak load demands. In that case, you'd need to add some additional output capacitance to cover the peak current demands.
You should also check out the design support area of our website: http://www.powerint.com/en/design-support/reference-designs/design-examples
There are numerous LinkSwitch-II design examples that would likely be a good starting point for your application.
-The Traveler
Can you give me a better idea of the power supply specs you require?
Important specs that come to mind:
Output Voltage - not clear from your question if it's a 5V or 3.8V output.
Output Current - Continuous output current (during CC mode for charging the battery)
Output Ripple
Any other important specs besides the peak load current of 2A?
-The Traveler