I have a DC-DC SMPS converter which is used as a pre-regulator in the power supply with linear post-regulator. A specific requirement is to provide mode of operation when pre-regulator is bypassed. One possibility to achieve that is by disabling it and in the same time activate mosfet which is connected in parallel with the pre-regulator to bypass it (see picture below).

Mosfet switch for bypassing

Another possibility is to keep pre-regulator active but with 100% duty cycle which effectively connect input (DC in) to output (Preg Out). The problem is that DC-DC converter is using N-channel mosfet as a high side switch when you in the best case could comes close to 100% duty cycle since boot capacitor which is used for gate bias need some "OFF-time" to be recharged. Here I comes to idea to "reuse" high side mosfet that is already a part of the pre-regulator as a switch mentioned in the first proposal.

Relay switch for gate bias

In this case gate driver output will be disconnected (using small relay) and continuous gate bias voltage will be applied to ensure continuous conduction. Simultaneously with this operation pre-regulator enable input will be disabled. A whole operation is not time critical, and post-regulator circuit should not have any problems with handling higher input voltage especially because "bypass" mode will be MCU-controlled to not exceed allowed power dissipation (i.e. 20W).

My question: Is it possible to do something like this "mechanically" or I need to do that is some other way because this could cause damage of DC-DC controller chip (I'd like to do this with LM5118 or LM5088)?

There is some proposal to deploy charge pump to keep boot capacitor timely charged (i.e. TI's slva444 - Providing Continuous Gate Drive Using a Charge Pump) but I'm not sure that such approach is possible with higher input voltage like 50+ VDC.


The trouble with the approach of dual-using the N channel MOSFET from your switching regulator is that the gate needs to be taken probably at least 5 volts higher than the source to switch it on reasonably.

This is achieved in normal operation because of the bootstrapping (a small amount of switching energy is stolen from the output to produce a power rail that is a few volts higher than your input power rail). The bootstrapping relies on the output regularly switching and charging a capacitor up. Once the output becomes DC, there is no possibility of this idea working.

However, I don't really see that the relay contact couldn't just be placed in parallel with the MOSFET - in effect you short the MOSFET out.

  • \$\begingroup\$ Yes this is true. I'll "lost" ~5V or need a charge pump in place. Thanks for suggestion with parallel power relay. The whole idea is in fact developed to avoid use of power relay since it's more expensive and require more PCB area. There is possibly an another approach: to use hot-swap controller that even in combination with additional n-mosfet cost less then reliable power relay. \$\endgroup\$ – prasimix Jan 28 '15 at 16:11
  • \$\begingroup\$ What current should be assumed? \$\endgroup\$ – Andy aka Jan 28 '15 at 19:02
  • \$\begingroup\$ Max. 3 or 5A (with power limitation of 20W). \$\endgroup\$ – prasimix Jan 28 '15 at 21:13
  • \$\begingroup\$ If you were considering a relay before, what current could that handle and, bear in mind, that a single pole single throw relay is all that is required - the K2 section can be a simple transistor and very likely the transistor that activates the coil itself. \$\endgroup\$ – Andy aka Jan 28 '15 at 21:46
  • \$\begingroup\$ Ok, initial idea was to use small signal relay such as IM03TS with 2A per contact is enough to handle charging of high side mosfet gate. I'm agree that K2 section could be a simple transistor. I just used relay since it has pair of contacts. Whatever I think that I'll take another route that will generate another question :) \$\endgroup\$ – prasimix Jan 28 '15 at 22:05

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