Designing MCU applications for use in high voltage environments: Part 2 - Embedded.com

Designing MCU applications for use in high voltage environments: Part 2

As mentioned earlier in Part 1,it is often necessary in a high voltage MCU design to create avariable pass resistance, reduce the variation in load current, orreduce variations in V UNREG to find a pass resistor'svalues that will work for all load and supply conditions. Here are someideas on how to implement such designs and the situations in which theywill be most useful.

Tip #1: Load Resistors.
One method for decreasing variations in the load current is to add loadresistors to unused microcontroller outputs (see Figure 4, below ).

Figure4. Load Resistor schematic

The outputs are set high to increase the load current at times whenother portions of the circuit are only drawing minimal currents andpull low to free up load current when the circuit needs more operatingcurrent elsewhere. This allows the microcontroller to manage the load,reducing variations in the load current.

If the minimum resistor value is greater than the maximum, then thecombination of supply voltage variation and load current variation cannot be supported by a single resistor. To correct the problem, thereare three choices:

1. The variation in currentdrawn by the circuit must be reduced.
2. A system for varying thepass resistor is required.
3. The variation in V UNREG must be reduced.

Tip #2: Open Collector Drives.
Another method for decreasing load current variation is to drive highercurrent loads with open collector lowside drives (see Figure 5, below ). This removes thedrive current requirement from the VDD supply and pushes itonto the higher supply voltage, reducing variations in the loadcurrent.

Figure5: Open Collector Driver Schematic

Tip #3 Open Drain Drives.
This method for decreasing the maximum load current is basically thesame as TIP #2, with the exception that the gate drive of the MOSFETdoes not require a continuous bias current (see Figure 6, below ). In addition, theON voltage drop across the MOSFET is significantly lower than a BJT.

Avoiding a continuous base bias current further reduces the loadcurrent on the system, and lowering the ON voltage of the drive meansthat more of the energy is actually delivered to the output. However,there is a drawback to using a MOSFET transistor. A current is requiredto charge or discharge the gate capacitance each time the transistor isturned on/off.

Depending upon the switching speed required, the current needed tocharge or discharge the gate capacitance can be as high as severalamps. This requires a MOSFET driver, which further increases thecurrent requirements of the circuit. Therefore, the decision to use aBJT versus a MOSFET should be based on the trade off between the biascurrent/on voltage advantages and the additional current requirementsfor switching the MOSFET.

Figure6: Open Drain Driver Schematic

Tip #4 Triac Drives.
In systems controlling AC powered loads, a TRIAC is often employed asthe output drive (see Figure 6, above ).However, while TRIACs have been available for many, many years, someengineers are still unfamiliar with their operation and do not takefull advantage of recent improvements in their design.

One of the most significant improvements in the TRIAC design is theintroduction of “sensitive gate” TRIACs. These devices are designed totrigger on a much smaller gate drive current than traditional devices.The reduced drive current also means that these new TRIACs will workwell, not only in quadrants 1 and 3,but also in quadrants 2 and 4, expanding the drive options open to thedesigner (see Figure 7, below ).

Another improvement has been a reduction in the holding currentspecification for the device. The holding urrent is the minimum loadcurrent at which the TRIAC will latch on, removing the requirement fora continuous gate current.

Figure7: Triac Conduction Quadrants

The base current required to bias the transistor is still sourcedfrom the VDD supply. However, the current gain of the transistor makesthe required base current only 1/10 to 1/100th of the actual loadcurrent.

What sensitive gate technology means for shunt regulator poweredsystems is that the average current drive requirement for a TRIAC drivecan be significantly reduced.

Lower gate drive currents reduce the current that themicrocontroller must source or sink to turn on the TRIAC and lowerholding currents reduce the time the microcontroller must hold the gatedrive during each AC cycle. Operation in all four quadrants allow themicrocontroller to use either positive or negative gate drive currentsto trigger the TRIAC.

Tip #5: Using GPIO To Supply V dd
In systems with an external circuitry, it is often advantageous to beable to power-down unused circuitry to reduce current consumption. Ifthe current requirement for a given section is less than the drivelimitation of a GPIO (typically 20 mA), then the power for thecircuitry can be supplied through a GPIO pin (see Figure 8, below ).

This puts control of the circuitry power under software control,which can then power the section only when needed and turn it off whenthe circuit is idle. If the current drive for a given section isgreater than the drive capability of a GPIO, then a simple externalswitching transistor can be used to manage the higher current level.Another option is to use the GPIO to control the Shutdown or Enableinput of the active devices in the circuit (op amps, ADCs, Filters orDACs).

Figure8: External Circuitry Power Control

Managing multiple sections of the design in thisway allows thesoftware to power-up different sections at different times, reducingvariations in the current consumption of the system by eliminating anyoverlap.

Tip #6: Transient Current Needs.
Some transient current requirements can be handled by temporarilyoverloading the VDD supply. In this scenario, a temporary output drive,which exceeds the current capacity of the system power supply, isenabled. The additional current required to supply the output is drawnfrom the bypass capacitor in the power supply and the power supplyvoltage momentarily sags.

Once the output is disabled, the current draw is removed and thebypass capacitor charges to 5V again. The amount of droop in VDD can beminimized by increasing the size of the bypass capacitor for thesystem.


Tip #7: Variable Pass Resistor.
Another method for handling a wide variation in supply voltage and loadcurrent is to vary the resistance of the pass resistor used in thepower supply design (See Figure 9,below ). In the example shown, the pass resistor is bypassed by alower value resistance under software control.

When the GPIO is set high, the open collector drive (Q1) pulls thebase of Q2 low, which turns on Q2 and bypasses the pass resistor R1with a lower value resistance R2.

The result is more current available for the microcontroller and thecircuitry powered by VDD . When the circuit returns tolow-power operation, GPIO is pulled low, both transistors turn off andthe pass resistance is just R1, reducing the system current.

The system is designed as two separate power supplies, one using theminimum and maximum current at the lower Current mode. The second usesthe minimum and maximum current at the higher Current mode.

The resistor value selected for the lower Current mode is used forR1. R2 is chosen to create a parallel combination that is equal to thepass resistor value selected for the higher Current mode. This willalso increase the amount of time required for the system to recoverfrom the over load.

The challenge is to balance the amount of droop against the recoverytime. Equation 7, above isused to determine the amount of droop the system will experience, and Equation 8 above is used to determine theamount of time required for the system to recover from a drop in supplyvoltage. This configuration a way to solve the problem of a single passresistor which has a higher minimum value than the desired maximumvalue.

Figure9: Dual Value Pass Resistor Schematic

Tip #8: Increasing Current With ASeries Pass Transistor
An external pass transistor, driven by a microcontroller shuntregulator, can produce a higher current power source for externalcircuitry (see Figure 10, below ).In the example shown, the pass transistor acts as a series regulatortaking its reference from the microcontroller V DD ,plus the forward voltage of the diode and providing a 4.9-5.2V powersupply.

The only requirement on the pass transistor is that the device musthave a sufficient break down voltage specification to handle the supplyvoltage and sufficient power handling capability to deal with the powerdissipated while the supply generates the full output control.

Figure10: Pass Resistor Schematic

Tip #9: Decreasing Variations In V unreg
With A Secondary Discrete Shunt Regulator. In the previous tips, wehave examined methods for reducing variations in the load current andmethods for varying the value of the pass resistor. The only othervariable in the design equations is variation in the supply voltage VUNREG, so, this final tip demonstrates a method for limiting the variationbetween VU_MIN and VU_MAX.

This is accomplished by regulating the high voltage side of VSERwith another shunt regulator, this time in the form of a Zener diode. Figure 11, below , shows an examplecircuit., in which two resistors, RHI and RSER, form a voltage dividerwith the mid-point voltage clamped by the Zener voltage of the diode.

If VUNREG increases, the Zener diode conducts additionalcurrent-to-ground causing the voltage drop across RHI to increase wileholding the mid-point voltage relatively constant. This reduces thevariation seen by RSER and the second shunt regulator in themicrocontroller, simplifying the choice for RSER.

Figure11: Secondary Shunt Regulator

The design for the high-voltage shunt regulator follows the sameprocedure as the original regulator in the microcontroller. The highand low variations in VUNREG are documented, as are the highand low load currents required for the load (microcontroller, shuntregulator and additional loads). A Zener diode is selected based on aZener voltage roughly half way between VUNREG_MIN and VDD .


Once this information is collected, a minimum and maximum value forRHI can be calculated. Equation 8 andEquation 9 , above, are just variations of Equation 1 and Equation 2 in Part 1 with the Zener voltagesubstituted for VDD .Two things should be kept in mind when using this tip.

First, the breakdown voltage of Q1 and Q2 must be greater than VUNREG .Second, The current range of the Zener diode should be large enough toput the Zener diode beyond the knee in the diode curve, (see Figure 12, below ), but still wellwithin the maximum power rating for the device. The maximum diodecurrent should also be determined and documented.

Figure12: I-V Curve For A Zener Diode

If the value can not be found that falls between the two limits,select a higher power Zener, which will increase IZENER_MAX or lowerthe Zener voltage to a value closer to VDD . If a value cannot befound, or if the power dissipated by the Zener or RHI is toolarge, andthe other tips in this application note can not provide any relief forthe problem, then a switching regulator may be needed to ultimatelysolve the problem.

However, if a value is found for RHI and the Zener diode, the nextstep is to determine the variation in IZENER based on VUNREG and RHI . Equation 11 and Equation 12, below , areused todetermine both the high and low Zener diode current.


Given the minimum and maximum Zener diode currents, the minimum andmaximum mid-point voltage can be found from the I-V chart for the Zenerdiode. The Zener diode power rating will limit the maximum current thatthe diode can sink when clamping the mid-point voltage.

However, most Zener diodes also specify a maximum surge power, whichextends the power rating of the device for transient power dissipation.If the maximum value for VUNREG is based on transient jumpsin the voltage due to noise or switching of other systems, then asmaller Zener diode may be used provided the transient is withinmaximum power surge power rating for the diode.

Figure 12, above , shows anexample I-V chart. To find the minimumvalue, scale the curve such that the Zener voltage is equal to theminimum Zener voltage value specified for the diode and take the valuefrom the minimum Zener current estimate.

To find the maximum voltage, scale the curve such that the Zenervoltage is equal to the maximum specified Zener voltage and take thevoltage value based on the maximum current. The maximum and minimummid-point voltages are then used for VU_MIN and VU_MAX in the originaldesign Equation 1 and Equation 2 (from Part 1 ) to calculate RSER. Theremainder of the design then follows the original procedure from thebeginning of this series.

The Zener diode power rating will limit the maximum current that thediode can sink when clamping the mid-point voltage. However, most Zenerdiodes also specify a maximum surge power, which extends the powerrating of the device for transient power dissipation.

If the maximum value for VUNREG is based on transientjumps in thevoltage due to noise or switching of other systems, then a smallerZener diode may be used provided the transient is within maximum powersurge power rating for the diode.

To read Part 1 in this series, go to Thebasics of shunt regulators.

Keith Curtis is PrincipalApplications Engineer in the Security, Microcontroller and TechnologyDevelopment Division at MicrochipTechnology Inc. In this role, Keith develops training and referencedesigns for incorporating microcontrollers in intelligent power supplydesigns. Keith also sits on the PMBus development committee, andis chair of the PMBus development tools subcommittee.

References:
1) AN1035 Designing with HVMicrocontrollers
2) AN786, “Considerations forDriving MOSFETs in High-Current, Switch Mode Regulators” (DS00786).
3) AN898, “Determining MOSFETDriver Needs for Motor Drive Applications” (DS00898).

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