Designing MCU applications for use in high voltage environments: Part 1
When designing embedded microcontroller applications, one of the greatest challenges can be the creation of the power supply for the microcontroller, particularly when the only supply voltage available is significantly higher than the microcontroller's maximum VDD.
Such situations arise in a variety of applications including white goods appliances, automotive applications and intelligent Point Of Load switching power supplies.
In an appliance or white goods environment in which a transformerless power supply is often used, the typical approach to powering a microcontroller off of AC is to step down the voltage to 8-10 VAC using a transformer, then rectify, filter, and regulate to the required 5 VDC.
Unfortunately, cost restrictions prevent the use of the transformer, and linear regulators cannot withstand 100+ volts, so the reduction of the AC is handled by a pair of series resistors. The AC is then rectified, regulated with a zener diode and filtered. Using a shunt regulator as the zener diode eliminates the cost of one more component in the transformerless power-supply design, and the regulator is also more accurate than the zener.
In automotive designs, the battery voltage generally runs a 10-13 VDC, with load-dump spikes as high as 40-50V. Load dump is the voltage spike out of the alternator, when the headlights or some other high-draw load is turned off.
There are regulators that are designed to handle this high voltage spike, but they are not cheap, and they are typically designed for 1 or more Amps of load current.
Using a shunt regulator, two equal-value resistors are placed in series between the battery voltage and the regulator. At the junction between the two resistors, a zener diode is placed as a voltage clamp. Typically, the zener voltage is chosen to be 2-3 Volts higher than the normal voltage at the resistor junction.
Now, when the load dump spike comes through, the zener clamps the center point and prevents the spike from passing through to the regulator. This is less expensive solution to a high voltage spike in the supply.
Another good example of an application in which a significantly higher supply voltage is used to power a microcontroller is the bias voltage generator in an intelligent Point Of Load switching power supply.
Here, the switching power supply needs to have some bias current to power both the PWM circuitry and the microcontroller, so it can accept control communications prior to powering up the main power-supply output, and for the initial switching to generate the output voltage.
Given that the intermediate voltage is typically in the range of 24 to 48 VDC, a simple linear regulator can't handle the job. As the microcontroller is already present, it can handle soft start, power up sequencing, error handling and communications. Combining the two functions together saves component cost, and expands the feature set of the POL regulator. Typically the best and quickest way to solve such problems is through the use of a shunt regulator (Figure 1, below).
|Figure 1: A typical shunt regulator design using a Texas Instruments TL431|
Current is supplied through the +24V input, and load current leaves through the +5V output. R1 is chosen such that, at the maximum 5V load, the drop across R1 is approximately 19V or a little less. If the 5V load is not drawing its full current, the voltage drop across R1 is smaller and the +5V output increases in voltage.
The shunt regulator senses the rise in the 5V voltage through the resistor divider formed by R2 and R3, and shunts current from the 5V line to ground to compensate. Using this system, the regulator acts as a variable resistor, adjusting its own value, such that the voltage output at 5V remains constant.
The regulator's ability to shunt current to ground only limits the variations in the system. If the +24V increases, the regulator will have to shunt additional current to increase the voltage drop across R1. If the load current on +5V decreases, the shunt regulator will have to shunt additional current to maintain the voltage drop across R1.
Both changes result in variations in the shunt current. If both variations are sufficiently restricted such that the shunt current remains within the regulators capabilities, then the actual magnitude of the load current is not important. However, regulating more than a couple of 100 milli-Amps would require both the +24V and the load current to remain relatively constant.
A simpler version of the circuit can also be built with a zener diode (Figure 2, below), which takes over the function of the shunt regulator, shorting current to ground whenever the voltage across the diode exceeds its zener voltage.
|Figure 2. Zener diode replacing the function of the shunt regulator.|
However, in many designs a more useful and flexible way to solve such problems is to integrate such shunt regulator circuitry onto the same die as the microcontroller that is typically also used in such applications. The advantages of a shunt regulator include:
1. Simple design, all that is needed is a resistor and a bypass capacitor.
2. No additional pins on the microcontroller, just power and ground.
3. Operating from voltages greater than 20 VDC is possible without special (and expensive) regulator circuits.
4. The resistor and bypass capacitor form an RC low pass filter, which helps to limit noise from the source and conducted noise from the microcontroller.
5. It eliminates one or more components from the power-supply design.
6. The supply voltage can be used to power other components in the circuit.
7. The amount of current available from the supply is not limited by the capabilities of the regulator. The regulator's current capability only limits the variations in supply voltage and load current.
8. For designs with large variations in supply voltage or load current, additional tricks can be used to keep the shunt regulator within its specified limits.
In certain of its microcontroller offerings, Microchip, for example, has incorporated an on-chip 5V shunt regulator. When compared to the use of a circuit such as the TL431 or a zener circuit, the advantages stack up pretty quickly in favor of a high voltage microcontroller incorporating the shunt regulator.
The TL431 requires two resistors, in addition to the TL431 itself, whie the zener circuit requires the zener diode. The HV microcontroller, on the other hand, only requires R1 and C1. It is more accurate than the zener circuit and it takes fewer parts than zener or the TL431. Plus, it doesn't require any additional pins on the microcontroller, the only pins used are ground and power.
The inclusion of an on-chip regulator allows the microcontroller to operate from a wide variety of supply voltages. As an added bonus, the shunt regulator topology also allows the connection of other circuitry, external to the microcontroller, to be powered by the VDD pin.
Whether externally, or as a part of the microcontroller's internal circuitry, the inclusion of a shunt regulator can simplify the design of control circuits which must operate from voltages above the normal range of the microcontroller's supply voltage. The circuit can even act as a supply for other active devices in the circuit. All that is required is a little careful design and component selection.
The basics of a shunt regulator
A shunt regulator generates a specific supply voltage by creating a voltage drop across a pass resistor RSER. The voltage at the VDD pin of the microcontroller is monitored and compared to an internal voltage reference (See Figure 3, below)
The current through the resistor is then adjusted, based on the result of the comparison, to produce a voltage drop equal to the difference between the supply voltage VUNREG and the VDD of the microcontroller.
The advantage to a shunt regulator is that the supply voltage, VUNREG, is only limited by the power dissipation and breakdown voltage of the external resistor, RSER, not the power or breakdown characteristics of the regulator. The challenge in designing a shunt regulator circuit is choosing an appropriate value for the resistor such that the range of currents over which the regulator has control will produce the correct voltage drop needed to produce a 5.0 VDC supply.
So, all we really need to know to design with a shunt regulator is Ohm's Law. The problem is that the supply voltage, VUNREG, is not constant and neither is the load current. In addition, the range of current over which the regulator has control is also limited. So the choice of RSER really becomes a balancing act, trying to find a resistance that will meet all three requirements.
|Figure 3: Shunt Regulator Block Diagram|
The best place to start in the design process is to catalog the variations possible in the supply voltage and the load current. For our purposes, the following definitions will be used:
VU_MIN is the minimum supply
voltage to the system.
VU_MAX is the maximum supply voltage to the system.
ILOAD_MIN is the minimum load current, excluding the regulator.
ILOAD_MAX is the maximum load current, excluding the regulator.
Given these values, it is now possible to determine the minimum and maximum pass resistor values for the circuit. Equation 1 and Equation 2, below, are used to calculate these values.
These values, RMIN and RMAX, represent the limits for the resistance of the pass resistor. The constant 5.0 V referred to in the equations is to the VDD voltage of the regulator, the 4 mA constant is the minimum regulation current for the regulator and the 50 mA constant is the maximum regulation current for the regulator.
If the minimum value is less than the maximum value, a final pass resistor value can be chosen between the two limits. Good design practice is to then check the minimum and maximum regulator currents. Equation 3 and Equation 4, below, show how these values are calculated.
The minimum regulator current must be less than the maximum load current and the difference must be less than the maximum regulator current of 50 mA. If not, then check the calculations for the pass resistor value. The minimum power rating of the pass resistor can now be calculated using Equation 5, below. Remember to allow for adequate cooling and an appropriate amount of margin when deciding on the final power rating.
Don't Bypass The Bypass Capacitor
The next step is to determine the appropriate size bypass capacitor for the design. While most microcontroller applications can use "rule-of-thumb" values for their bypass capacitors, the unique nature of the shunt regulator complicates the selection.
First of all, the combination of the pass resistor and the bypass capacitor form an unintended RC time constant that limits the rise time of the microcontroller VDD. Therefore, it is necessary to limit the size of the capacitor such that the resulting rise time for the VDD supply is faster than the specified minimum rise time for the microcontroller's VDD.
For example, the minimum rise time for a Microchip microcontroller's VDD is 0.5V/mS, until the supply voltage exceeds 2.1V (Power-on Reset trip point). So, the supply must exceed 2.1 volts within 42 mS, (2.1V/0.5V/ms). Using this information and Equation 6 below, the maximum capacitor value can be determined.
The bypass capacitor must be less than the value specified by Equation 6 to meet the power-up requirements of the Power-on Reset, and greater than 0.1/.047 microfarads for noise suppression. Typically, the value is chosen to be closer to the 0.1 microfarad value for convenience.
Next in Part 2: Tips and tricks for implementing shunt regulators into high voltage MCU designs
Keith Curtis is Principal Applications Engineer in the Security, Microcontroller and Technology Development Division at Microchip Technology Inc. In this role, Keith develops training and reference designs for incorporating microcontrollers in intelligent power supply designs. Keith also sits on the PMBus development committee, and is chair of the PMBus development tools subcommittee.
1) AN1035 Designing with HV Microcontrollers
2) AN786, "Considerations for Driving MOSFETs in High-Current, Switch Mode Regulators" (DS00786).
3) AN898, "Determining MOSFET Driver Needs for Motor Drive Applications" (DS00898).
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