PMP8740: Pre charge

Dear Dinesh,

Sorry for the late reply.

Below is shown a list of anwers to your questions:

1) The pre-charge circuit in PMP8740 has been designed to limit the current flowing from battery to power supply when a battery is connected and all electrolytic capacitors are still discharged. For this reason there are two thermistors as precharge way. After the battery precharges the electrolytics, the series FETs can be turned on, and the DC/DC converter starts in a "pre-biased" way (residual voltage present)". The whole circuit is better explained on page 37 of the white paper, describing the power mudule, downloadable at the link:

The precharge current is simply the battery voltage divided by the value of RT2

2) The precharge current can be adjusted by selecting the value of those thermistors (RT1 for reverse polarity current limit and RT2 for precharge)

3) If you don´t have any switching voltage, that can be used as charge-pump, you may use an inexpensive push-pull converter and small transformer, by employing a SN6501

4) The precharge MOSFETs can be driven off, by using a zero-logic voltage to the digital line "Pre-Charge P2.2". There are other way to add precharge circuitry on a DC/DC converter. In our WEB site, we have or-ing controller, reverse polarity protection IC etc. Depends on the output voltage of your project, and the function you want to implement, there are some solutions. Please let me know which funtion is necessary in your case.

Best regards,

Roberto

Hi Dinesh,

I do not suggest to use high voltage (here up to 58V) charge pump ICs, but just to add a small SN6501 + transformer; please visit the product page for accessing the documentation and application examples.

By the way, I see here that you need a maximum current of 8A, so probably all parallel MOSFET I used are not necessary.

If you don´t need any reverse voltage protection (which happens by misconnecting the battery), but you want to use reverese current protection, you may want to consider the LM5050 (www.ti.com/.../lm5050-1.pdf, which is an or-ing controller. This controller avoids reverse current flow from the load (the battery) and the output electrolytic capacitrors of the DC/DC stage. At the time I designed my PMP8740, this chip was not available. After you enable the DC/DC stage, the voltage on the electrolytic capacitors will ramp up slowly (thank to soft start) and when the ouptut voltage of the DC/DC stage is > of the battery voltage, the LM5050 will activate the external MOSFET to reduce the losses in the internal body diode (it´s a sort of "active diode").

You can use also MOSFETs at lower side, and then you don´t need any charge pump. I didn´t use this method, because my power supply was part of  modular system, therefore I needed my system to have common ground.

Best regards,

Roberto

Hi Dinesh,

I am not sure we have so high current switch controllers, but you may have a look to the product page of them:

BTW, can you please double check if you are allowed to connect two batteries in parallel by means of a switch (relay or solid state)? I mean, if they are disconnected and they have sligltly different charging state (and voltage), when connected together a high current can flow from the higher voltage battery to the lower voltage, without control. If you use a linear hot swap IC, it will have high losses in the swithcing elements, because it has to work in linear region. Probably you need to use active balancing circuitry.

Best regards,

Roberto

Hi Dinesh,

In this case I suggest to have a look to our hot-swap product line, like:

TPS2490 (-91, -92, -93), LM5066 and LM5069

All of them drive external FETs, which should be fine for the solution you are looking for.

Best regards,

Roberto

Hi Dinesh,

I close the thread by now, but feel free to reopen it in case you need more support.

Best regards,

Roberto