I am trying to come up with a simple Battery Charger design, which utilizes PWM control. The use of high frequency PWM is to reduce the size of the Transformer that is to be used. Planning to use Transformer to provide the much required Galvanic Isolation between the AC input and the DC Charging output.

Specs of the Transformer are roughly: Input: 0-220V Output: 0-18-36-72 (only 1 tapping shown here). Transformer primary is driven by a PWM power signal at approx 25KHz. This choice of frequency comes from an application note that the MCU manufacturer has provided. PWM generated from a PIC16F, and will be optically coupled to the MOSFET gate. The values of C and R shown below are only indicative. Battery temperature is monitored separately and fed to the controller. Also, Battery presence etc are taken care of separately. Vsense is carried out during PWM-OFF cycles, while Isense is done during PWM-ON cycles.

I have few important questions:

  1. Primary will already have DC, being switched. Do I still need a rectifier on the secondary? A polarity limiting diode shall be present anyways.
  2. I plan to switch the primary, with a single MOSFET. What kind of MOSFET should be used and also how to connect it. I have used an N-Channel Enhancement type. Is this correct?
  3. Some designs show an output L-C filter, with L in series and C in parallel to the Battery? Do I need something similar? If yes, Why?
  4. The smoothening CAP that I have used on the primary side...what is a good approximation for its capacitance value?
  5. What other precautions should be kept? I wish to know if there is something important lacking from this power schematic..something that I might have overlooked.

Please Note, I understand that this is by no means the complete schematic. It would be impossible to provide everything here. My basic question is about the feasibility of the power scheme, and if any additions MUST be done for a viable design. Would help a lot if you could point the glaring issues out.

All help/comments etc are highly welcomed.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ 1. Do you want to charge your battery with AC? 2. You are way over your head here if you need to ask that. Why don’t you start with a 50/60 Hz iron step down transformer and try making a buck battery charger on the secondary in order to learn? 3. How much ripple are you willing to feed your battery? 4. About 0.5 uF per watt output. 5. Don’t play with mains voltage! \$\endgroup\$ – winny May 5 '18 at 10:35
  • \$\begingroup\$ @winny: (3). So my understanding was the Battery will act has a huge capacitor, when it comes to ripple. Will the battery manufacturer specify this ripple value? This is for a regular Sealed Maintenance Free Lead Acid Battery, 12V, 100AH. \$\endgroup\$ – Vishal May 5 '18 at 11:38
  • \$\begingroup\$ @winny: (4) Ok, so 0.5uF per watt. Thanks, that's a very good data point. Thanks for your answers. We already make Battery Chargers, which use SCRs commutating over line frequency and feeding a regular line frequency transformer. The problem is Transformer size. I wish to draw upto 20A, at 24V and some extra budget. This leads to a Transformer close to 600VA. Transformer size is quite big. Now, if the frequency can be increased, the inductance required will reduce. So I want to use the present scheme. Also, Thanks for the safety advice. We take due care in dealing with mains. \$\endgroup\$ – Vishal May 5 '18 at 11:41
  • \$\begingroup\$ @winny: (1). Of course, I want to charge my Battery with DC, but in this case, Transformer primary is being fed with switched DC...which moves between 0 and 220*1.414. The transformer secondary will switch between 0 and 18*1.414. Would that constitute AC? \$\endgroup\$ – Vishal May 5 '18 at 11:45
  • \$\begingroup\$ @winny: (2). Its just that I have not worked with MOSFETs that much. I use SCRs or Triacs more often. \$\endgroup\$ – Vishal May 5 '18 at 11:45

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