I am designing a hybrid power supply. The system is such that it will house an LDO and a switching regulator as well. The system has long intervals of sleep mode. Every now and then it wakes up and transmits/receives data. Its the transmission/reception times that a lot of current is drawn and the SMPS kicks in.
I intend to make use of a system whereby the current consumed is measured and the relevant supply is switched. One of the ways I can do that is using a current sensing resistor. The current is measured using an ADC and we turn on/off relevant MOSFETs that channel power from the LDO/regulator.
Now, my concern is about the MOSFETs. The MOSFETs need to be very high speed as the power source must be switched from LDO channel to switching regulator.
What is the average switching time for a MOSFET ? Also, can I speed up the switching time of a MOSFET by manipulating the gate voltage ? I understand that gate capacitance is an important parameter here.
Is my plan of action of using MOSFETs for this a good approach ?
Kindly advice.
Best Answer
A MOSFET gate can be modeled as a capacitor. "Manipulating gate voltage" is not going to make it switch faster. Rearrange \$i(t)=C\dfrac{dV}{dt}\$ to \$t_r\approx \Delta t=\dfrac{C\Delta V}{I}\$. As you can see, to get rise time down, you need a MOSFET with low gate charge, and you need to push as much current into the gate as possible for a short period of time. This is usually done with careful selection of a MOSFET, and the use of a gate driver IC. This type of configuration easily yields sub-nanosecond rise times.
The reality is you probably don't need the MOSFETs to switch as fast as you think. With some capacitance on the common switched node, you can probably tolerate fairly long switching times, but it's impossible to say how long from the information given. Further, what is the point of the LDO when you have a SMPS that can supply the system? When it supplies the system, it's wasting power.