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I want to create an ENABLE signal driving a Murata OKI-T/3-W32P-C DC-DC converter, where the P stands for Positive Polarity of the On/Off Control line: the line has an internal pull-up, which means that the converter is on when the Control line is open or high, and off when pulled to GND.

So what I have right now is a DISABLE signal, based on an N-channel enhancement-type MOSFET (2N7000). initial schema

From what I read online, the easiest way to turn this setup into an ENABLE signal would be to use a depletion-type MOSFET, but I understand that these are rarely used? Or are there depletion-type MOSFETs that are generally recommended just like 2N7000 and other general-purpose transistors?

If not, what would be the right way to go here? connecting the DISABLE signal to Vin using a pull-up resistor, and add another transistor at the low side to pull down the DISABLE signal when the ENABLE signal is activated?

Follow-up:

So here's the adjusted schema following @peufeu solution (thanks for providing a complete solution + explanation): adjusted schema

Which leaves me with the question whether this is the general way to go, or whether the MOSFET-based solution can/should be used when Vin is not as high as 32V:

In the datasheet for this OKI-T/3-W32P-C module I see that there is simply a switch symbol. But in the datasheet for the ABB ABXS002A3X41-SRZ module (fig. 27 on page 11), for example, there is a N-channel/enhanced MOSFET in the circuit. Is that because the max Vin in this case is only 16V? which means that a 2N7000 could be used directly here, as its max Vgs is 20V continuous? ABB ABXS002A3X41-SRZ ENABLE setup

Similar thing in the datasheet for the Murata OKL-T/6-W12 module: Fig. 7 on page 15 shows the P-channel/depletion setup (for a module with negative logic) that I initially thought I could use. This module has a max Vin of 14V, so in this case a MOSFET would be able to handle it without level shifting? And if so, what transistor would be a good candidate? Murata OKL-T/6-W12 ENABLE setup

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  • \$\begingroup\$ adjusted the title, as my initial setup was wrong \$\endgroup\$ – Adrian Sep 14 '19 at 18:11
  • \$\begingroup\$ I've edited my answer to reply to your edits \$\endgroup\$ – peufeu Sep 15 '19 at 14:10
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Quick datasheet check...

enter image description here

It does not use standard logic levels: "Low" is below 1.2V, and "High" is above Vin-2.5V.

So, level shifting is required if you intend to use standard logic levels (like 3.3V CMOS) to control it.

the easiest way to turn this setup into an ENABLE signal would be to use a depletion-type MOSFET

A depletion N-Channel MOSFET has a negative threshold voltage, it is ON with Vgs=0V. However, it is still N-Channel, which means in order to turn it OFF, you need Vgs to be below the threshold, which means a negative Vgs. I don't think this is what you want, if your control signal is standard logic, it will be positive.

So the simplest solution is to use 2 transistors to invert the signal twice and level-shift it. They can be bipolar or FET. Since the datasheet specifies an iternal pullup on your DC-DC it would be like this:

schematic

simulate this circuit – Schematic created using CircuitLab

I used BJTs because your 32V max input voltage will blow the gate of most FETs, so they would need extra components to limit Vgs. I added a pulldown on the input too. This is not necessary if the input is connected directly to a logic circuit, but if it is a connector it is better to have a pulldown to avoid spurious turn-on if a foreign object (ie, a finger) injects some tiny current into the base of the transistor...

in the datasheet for the ABB ABXS002A3X41-SRZ module (fig. 27 on page 11), for example, there is a N-channel/enhanced MOSFET in the circuit. Is that because the max Vin in this case is only 16V? which means that a 2N7000 could be used directly here, as its max Vgs is 20V continuous?

In the ABB case fig 27: FET gate is connected to ENABLE signal you provide -- thus you must pick a FET that is compatible with the logic levels of your ENABLE signal, it must fully turn ON on a logic high (5V or 3V3 etc). In this case it is up to the circuit that generates ENABLE signal to ensure Vgs will not exceed 20V. If it comes from a logic circuit, or a switch from the 16V supply, no problem.

Similar thing in the datasheet for the Murata OKL-T/6-W12 module: Fig. 7 on page 15 shows the P-channel/depletion setup

The schematic shows a weird MOSFET symbol with incorrect connections and a short between base and emitter of bipolar transistor Q3... ie, this schematic is most likely wrong, don't look at it, follow the instructions in the text instead!

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