Hall Effect Current Measurement

Hall effect current measurement isn’t a direct measurement of electric current.  Rather, the hall effect is caused by the magnetic fields that electric currents produce.  Hall effect describes a change in resistance of a very thin conductive film when it has a magnetic field passing through it perpendicular to its surface.  Thus, the hall effect is a direct measurement of the magnitude of a certain orientation of magnetic field.  When the thin film is properly situated near a larger conductor, the magnetic field measurement can be turned into an accurate measurement of the current through the larger conductor.  Reading unwanted nearby magnetic fields is possible, and strong magnets can saturate the hall effect sensors, but shielding measures can be taken to prevent those sources of error if necessary.

So the thin film is placed near a larger conductor, and the hall effect causes the resistance of the thin film to increase as a function of the magnetic field arising from the current through the larger conductor.  This resistance is then converted into an output voltage by passing a separately sourced current through it, the signal is given low source impedance with a follower and/or amplifier, and we get an accurate measurement of the current without being in direct electrical contact with the conductor being measured.  This ability to measure current without contact is highly advantageous in situations where the measurement has to be made at a portion of the circuit that is at a high voltage with respect to ground, and the current source can be designed to provide as much accuracy as necessary as well as providing outputs that are linear with respect to current.

Shunt Type Current Measurement

Shunt type current measurement is a pretty simple method.  It is effectively just measuring the voltage drop across a known resistance, and applying Ohm’s law (V=IR) to calculate the current.  But, in practice, there are a lot of issues with this.  First and foremost is that higher voltage drops are easier to read, but the higher the voltage drop across the sense resistor, the more power is wasted to heat in the sense resistor, and the more heat, the larger the resistor needs to be.  It is possible to gain some current data from a sense resistor with a maximum voltage drop of 100mV, but expensive, sensitive, and highly accurate instrument amplifiers are required, and it still may not be possible to tell the difference between, say 3/4 of the maximum current and 13/16 of the maximum current.  So to use shunt type current measurement you either have to waste significant power in an expensive and large sense resistor or rely upon expensive instrument amplifiers which can easily be sensitive to thermal or other sources of electrical noise.  But that’s not all.  Shunt type current measurement also inherently suffers from non-linearity due to temperature changes because no resistor material has constant resistivity with respect to temperature.

Broadcom ACHS-7123

While taking my BOM to Mouser to plan a parts order, I found that one of my preferred board mount current sensors, the Tamura L18P030S05R isn’t currently available in low quantities.  Fortunately, there is an equivalent hall effect type current sensor that comes in a smaller, cheaper smd component, the Broadcom ACHS-7123.  I prefer this technology over shunt type current sensors, as I think that they are, overall, cheaper, more accurate, and smaller, and this “new to me” Broadcom chip is a fine example of that.

Simplifying my 24VDC ESC, VFD, Inverter

I’ve been working on simplifying my 24VDC inverter design using the MIC4605-1YM in place of my individual component gate drivers, and I’m revising the layout with more layers, a switch and pot, smaller PCB area, and aligned and grouped SMD components for pick and place.

Anyway, while perusing the data sheet for the MIC4605 again, I discovered that the MIC4605-1YM’s shoot through protection isn’t quite the same as what I think the L6498 has.  The shoot-through protection built into the MIC4605-1YM adds a long (relatively) 240ns delay when it is activated.  This means that the protection can’t be relied upon to permit one to drive the high and low inputs intentionally with “wrong” but more efficient signals.  As a result, I’ve had to add in a couple of fast quad logic gates to implement what is effectively an instantaneous shoot-through protection logic ahead of the inputs of the MIC4605-1YM.  This is a little disappointing and it eliminates the cost and size benefits of the 4605 over the L6498, if the L6498 operates with delay-less shoot through protection logic.  The caveat to this is that the L6498 datasheet doesn’t explicitly state that the overshoot protection is delay-less, and I’ll have to test it to confirm that they have the functionality I expect.

ESP32 and Psoc 6

There are two system on a chip devices that I’ve been looking at for more advanced on board processor applications. These devices have a good amount of flash memory for programming, fast, 32 bit processors, ample ram, and most interestingly, wifi and bluetooth interfaces built into a single chip. Two of these are the Cypress PSoc 6 and ESP32. The ESP32 surpasses the PSoc 6 in a few specifications such as almost twice the ram, and the ESP32 has a packaging option that includes 2MB of flash memory. But otherwise, the chips are comparably capable. So another thing to consider is ease of development. The ESP32 can be developed in the arduino IDE, which promises good community support, but the basic arduino programming language wasn’t intended for the ESP32, so additional open source libraries are required to unlock the esp32’s full functionality. The Psoc 6, on the other hand has companion windows development software designed specifically for the chip that is supported by the manufacturer.

The big differences between the systems comes in packaging. The ESP32 is increasingly available in mini modules such as the WROVER which include expanded memory for use as mini networking computers. But this module comes in a 20 pin package, severely limiting the parallel I/O capability. The psoc 6 on the other hand is available in a BGA116 package with 78 I/O pins or is also available in their WICED modules with built in antenna to provide Bluetooth tx/rx capabilities comparable to the WROVER but the WICED modules lack the wifi and added memory, and are more expensive.

All in all, it looks to me like the ESP32 and its modules have an advantage for now for most simple networking applications, with cheaper, more capable modules, and I’m going to focus on it for those.

ST L6498LD 500V Gate driver

This half-bridge high and low transistor gate driver solution can be used for higher DC voltage power buses, up to 500V, and is faster for transistors with higher input capacitance.  The only drawbacks when compared to the Micrel MIC4605 or MIC4604 are that the Micrel chips are available in smaller packages then the L6498 is available in because the L6498 can pass more power to the transistor gates, and the Micrel chips are presently cheaper.

For now, the L6498 is my choice for general half bridge circuits switching voltages in excess of 80V.  This chip does not have a single PWM input, having instead separate high and low inputs, but this is actually a feature as the chip has built in shoot through protection, and the voltage control switching can be applied to the high side while the current direction can be applied to the low side.

 

Micrel MIC4605

I’ve been lately searching for the best half bridge driver IC’s for my reference.

A good, fast, and cheap half bridge driver for DC main voltages up to 85V is the Micrel MIC4605-2YM-TR.  This device requires only two additional capacitors to work, and is triggered by a convenient single PWM input.  The device ratings indicate that it should still be somewhat functional at PWM frequencies up to 1Mhz for driving power mosfets with gate capacitance of 1000pf or less, but no doubt operates with much easier to insure (easier to design a pcb for) reliability in the 50-200kHz range.

I still need to find the best IC for DC mains up to 300V, if such can be found.

The search has been educational.  For one thing, I’ve learned that the shoot-through protection on the output of these chips affects the output timing of every HO and LO cycle, basically holding HO low until LO is low which affects the usable PWM input frequencies and duty cycles by adding some dead delay time.

The gate drivers circuits and microcontroller of my inverter don’t need that shoot through protection on every PWM_IN cycle.  Rather, the shoot-through protection is only necessary when switching the FOR_!REV output.  Therefore, in the programming of the controller, the PWM_IN output has to be disabled for a few clock cycles while switching the FOR_!REV output from high to low or low to high.  This is a much smaller effect than on the MIC4605 for an inverter or VFD application because the 50% duty cycle PWM output for FOR_!REV is equivalent to the output frequency which will typically be between 50 and 2000Hz.

 

 

Gate Driver IC’s

There are a wide variety of half bridge gate driver IC’s available. These offer high and low transistor gate driving functionality compressed into small packages to limit PCB size and complexity. Most of them require a bootstrap capacitor which means that they have limitations to the combined PWM frequency and duty cycle (due to capacitor charge time, leakage currents, gate currents, and drain currents). They also need to be reviewed for acceptable delay characteristics. However, they offer a cheap and fast solution for most systems that have fixed output or limited variable outputs.

NMOS High Side Switches

In various switching power electronics, it is common to see four N channel enhancement mosfets connected in an H configuration with the load making the horizontal bridge and each mosfet making up half of the the long vertical sides.

There are a few issues with getting those high side mosfets to turn on. N channel enhancement mosfets become conductive when their gates are raised to a certain voltage above their sources. This is no problem for the low side FETs because their sources are connected to ground. So you just need to apply a 5-15V voltage with respect to ground and those FETS are on. But what happens if you try that with the high side? Well, once one of the low side FETs is on, the load gets pulled down to ground voltage on both sides. This lowers the source voltage on the high side FETs to a level where the 5-15V above ground is enough to turn it on. But there is a problem. If the high side FET turns all the way on like that, it’ll have such a small voltage drop from drain to source that the source voltage can be too high. About as high, in fact, as whatever the main high voltage is. And if this is higher than the 5-15V on the gate, that high side FET will start to turn off, but not necessarily all the way. It’ll stay on exactly enough to keep the source voltage low enough to stay as on as it is. This can easily burn up the transistor, depending on the load impedance. If the load impedance is 1 Ohm, and main voltage is 24VDC, the FET gate threshold is 3V, and the gate voltage is 10V, then the transistor needs to have enough impedance such that the voltage to the load is about 7V. Thus, the resistance of the transistor will set itself to around 3.4 Ohms. Trouble is, with that resistance at that voltage, the transistor has to dissipate 92.7 Watts which is enough to rapidly overheat and destroy any FET that doesn’t have some serious active cooling, and even if the FET can survive the heat, the load is still only getting 7V out of 24V, and only 38W of power. That’s no good.

One alternative is to use the high main voltage at the gate of the high side FET. In the last example, if you did this, the load would get closer to 21V, the FET still has to dissipate 63W, but the load gets a decent 442W. This is better, but it’s no where near as good as the FET can do.

The best method is to apply a voltage to the gate of the FET that is 5-15V higher than the high main voltage. This can be produced in a number of ways using switched devices, and if one is clever, and is driving the gates with a PWM wave, then one can use that switching to get the high side FET gate voltages up. The only problem with that is that if the duty cycle gets too close to 100%, then you end up right back in the second situation above, with massive heating of the high side FETs. A more flexible method would be to source the voltage for the gates from a separate boost converter with its own internal switching, but this is a more expensive choice.

Finally, there are a few P-channel power mosfets that can be driven a little easier, requiring only the high main voltage and a voltage 5-15V lower than that. This is problematic for two main reasons. Firstly, the timing is different. When using all of the same N channel FETs, you know they all have the same switching delays, with the P channels, you can expect different switching delays. Secondly, for the highest power electronics, P channel devices aren’t available (technically N channel FETs are also unavailable, but IGBT’s operate similarly to N Channel FETs).