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'[EE] High-side/Low-side MOSFET?'
I have a question for which I have not found a concise answer after
having searched the web. While researching the use of MOSFETs for
switching a load (in this case a resistive heater element) I have often
seen the terms "high side" and "low side" switching. There are
specialized gate drivers for each configuration, none of which I need;
I'm just curious about their use. I understand the terms high side and
low side as applied to an H bridge, but this seems to be a different
I have gathered that the "high side" is simply an inline MOSFET that
sources/sinks the load. Is that correct?
I have not come across an explanation of the "low side" switch. More
accurately, I have not found an explanation that was complete enough for
me to follow.
If anyone would be so kind as to explain these terms, or point me to
suitable reference material, I would appreciate it. Thanks!
- Glenn Minch
> I have gathered that the "high side" is simply an inline MOSFET that
> sources/sinks the load. Is that correct?
> I have not come across an explanation of the "low side" switch. More
> accurately, I have not found an explanation that was complete enough
> me to follow.
Low side means that the switch (here a FET) has one side connected to
High side means the switch has one side connected to the high voltage
(usually but not necessarily positive) supply.
An H bridge has both high side and low side switches so that each end
of the load can be alternately connected to low or high sides thereby
applying an AC signal to it.
With loads that need only mono-polar on-off DC applied you often use
only a single switch. Electrically it doesn't matter if it is in the
high or low side - either way the circuit is made or broken,
In practice it can matter a lot. Loads are often ground connected at
one end (or will ideally be ground connected at one end.) For example
a car tail-light bulb may be hard connected to ground via the car body
and you would normally switch the positive or high side to turn it on
and off. In such an example, if you DID switch the low side the bulb
body would float to +12v when off and would present a potential
hazard. Connecting an earth to the body would turn the bulb on. With a
bulb where +12 is switched
Also, in some cases the power supply is hard connected to ground at
one end and a switch cannot be placed in the ground or (usually) low
side lead. In such cases a high side switch is needed.
Low side switches are usually more convenient to drive as the main
electronics are usually low side (ground) referenced. So a FET that
needs eg 10v gate drive needs a 1v signal relative to ground.
A high side switch must have its drive signal referenced relative to
the high side. If high side rail is 100 volts this requires level
translation from the ground referenced electronics. Worse. switches
that need a positive signal relative to their low est voltage
connection (N channel FETS, NPN transistors) will need a gate or base
drive signal ABOVE the high side rail OR floating at the potential of
the top of the load - a voltage that may be anywhere from ground to
high side. This often requires making a special power supply that
floats relative to ground and which is then referenced to the FET etc
source. A solution to this need for a special hi side supply is to use
a PFET or PNP transistor high side switch which then uses drive
signals below the high side rail. P channel FETs are usually dearer
and/or with worse specs than the N Channel equivalents so many
applications use a high side gate drive supply.
|Russell McMahon wrote:
>> I have gathered that the "high side" is simply an inline MOSFET that
>> sources/sinks the load. Is that correct?
>> I have not come across an explanation of the "low side" switch. More
>> accurately, I have not found an explanation that was complete enough
>> for me to follow.
> Low side means that the switch (here a FET) has one side connected to
> High side means the switch has one side connected to the high voltage
> (usually but not necessarily positive) supply.
For some examples of such switches:
- http://www.irf.com/product-info/datasheets/data/ips5451.pdf A high side
switch. Look at the "Typical Connection" diagram on the first page: the
load is connected between the switch and ground, and the switch is
therefore on the "high side" of the load.
- http://www.st.com/stonline/products/literature/ds/7374.pdf A low side
switch. Look at Fig. 3 (Unclamped Inductive Load Test Circuit): the load is
connected between the switch and the high voltage rail, and the switch is
therefore on the "low side" of the load.
You also can see some of the differences Russell explained in the internal
circuits of these switches.
A simple switch component (transistor, MOSFET, relay etc) can generally be
used as both high side or low side switch, depending on how you connect it
and/or drive it. It's the additional circuit included in these ICs that
make them specific for either application.
> I have often
> seen the terms "high side" and "low side" switching. There are
> specialized gate drivers for each configuration, none of which I need;
> I'm just curious about their use.
In a simplistic circuit powering a switched load, there are two
configurations. One end of power supply connected to common, "high" end to
switch, switch to load, load back to common. The other configuration is
common, then power supply, then load, then switch. The difference is
whether the switch interrupts the high or low side load connection, with low
referring to the connection between the load and common.
Therefore "low side switch" in your context might refer to an N channel FET
with source tied to common and drain tied to the bottom end of the load.
The top end of the load can then be permanently connected to the high side
of the supply. A "high side switch" might be a P channel FET with source
tied to the high side of the supply and drain tied to the high side of the
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com
> -----Original Message-----
> From: mit.edu piclist-bounces
> [mit.edu] On Behalf Of Gerhard Fiedler piclist-bounces
> Sent: Saturday, August 13, 2005 5:49 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] High-side/Low-side MOSFET?
This is a little belated - I've been kind of busy & have gotten behind
on email - but I want to extend my thanks to Russell, Gerhard, and Olin
for their explanation of the high side/low side terminology.
I have to thank Gerhard for including links to the data sheets, partly
because it's good to have a diagram to illustrate the usage of the
concepts that Russell described, but also because he turned me onto a
couple of devices that I had no idea existed. The IPS5451 datasheet sent
me off on a search that uncovered a device which is ideal for my
application. (http://www.st.com/stonline/books/pdf/docs/7370.pdf) These
are only $2.28 each from Mouser. I have a couple of PWM applications in
mind, for which the VN820 is an excellent match. I doubt I would have
found that item if I hadn't asked my question here.
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