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Transformerless AC power supplies accept wall-outlet AC voltage (such as 120VAC) input and produce a low voltage DC (such as 3VDC) output. The low voltage DC is typically low current output, on the order of tens of milliamps. This Design Note explains how transformerless power supplies work and examines trade offs to consider when designing them. The options described herein can be implemented with the Proto Power Switch, a circuit for prototyping products that control AC voltage and/or are powered by AC voltage.
The Proto Power Switch can be ordered pre-populated or as abare board. Beyond the prototyping stage, can support transition to production by designing and sourcing custom circuit boards specific to your application.
Transformerless AC Power Supplies
Transformerless AC power supply theory is not generally taught at the university level, yet the use of such power supplies is prevalent in consumer goods. Online white papers and websites provide example circuits and equations to aid the design process, yet these sources emphasize the math without explaining the fundamental principles of operation. The aim of this Design Note is familiarize the reader with the basic concepts, such that the equations can be derived, constructed, and manipulated intuitively. The scope of this document addresses circuits which accept single phase AC voltage (e.g. 120VAC, 240VAC) and output low, fixed DC voltage.
Described simply, the AC input voltage charges up an output filter capacitor. The AC voltage is rectified to ensure that the capacitor is only charged and not discharged by the mains. Voltage division ensures that only a small fraction of the input voltage shows up across the output capacitor. Lastly, a Zener diode in parallel with the output capacitor performs basic voltage regulation.
Rectification
The first step in the process is to rectify the high voltage AC sinusoid such that the remainder of the circuit is only ever exposed to positive voltage. This is achieved by passing the sinusoid through either (1) a full bridge rectifier composed of four diodes, or (2) a single diode that blocks the negative part of the sinusoidal voltage, as shown in Figure 1.
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| Figure 1 |
Using a High Voltage to Make a Low Voltage
A well-known method of generating a low voltage from a high voltage is to use a voltage divider circuit, as shown in Figure 2. In textbook examples, the impedances Z1 and Z2 are typically resistors, and if only negligible current leaves through Vout, then the voltage we can expect at Vout is Vin * Z2 / (Z1 + Z2).
Using resistors for both Z1 and Z2 will generally result in a poor power supply design. Good power supplies support a range of output current from Vout while holding the output voltage constant. In a resistor-based design, if there is any load current then the voltage drop across Z1 will increase and Vout will correspondingly decrease, which is undesirable.
Another issue is that if Vin were to decrease, then Vout will decrease by the same proportion. Given that Vin varies significantly over time (see Figure 1), we know with certainty that Vout will also vary over time as well, which is again undesirable. To improve the performance of the power supply, we can replace the Z2 resistor with a Zener diode instead.
An ordinary diode will pass current in one direction (indicated by the arrow of the symbol) but will block current if it tries to flow the other way, preferring instead to allow a reverse voltage to build up across it. The Zener diode has the unusual characteristic that above a specified reverse voltage (the Zener voltage) for the device, it will no longer block current but instead allow current to flow through it in reverse. This reverse current flow occurs only when the reverse voltage across the Zener diode grows high enough
to match the Zener voltage rating.
Note that the Zener diode will actively dissipate power when reverse current flows through it because there is a voltage (the Zener voltage) across it. This power is P = I*V. Using a Zener diode instead of a resistor for Z2 produces a better power supply which maintains constant output voltage despite changes in input voltage or load current. Regardless of whether 1mA or 30mA is flowing through the Zener diode, its Zener voltage does not change (much). Thus, Vout also will not change, even if a load connected to Vout draws current (current that would have otherwise passed through the Zener diode).
If the small amount of inherent Zener diode voltage variability is unacceptable, Vout can feed into an LDO or DC/DC power supply, which will provide improved output regulation.
to match the Zener voltage rating.
Note that the Zener diode will actively dissipate power when reverse current flows through it because there is a voltage (the Zener voltage) across it. This power is P = I*V. Using a Zener diode instead of a resistor for Z2 produces a better power supply which maintains constant output voltage despite changes in input voltage or load current. Regardless of whether 1mA or 30mA is flowing through the Zener diode, its Zener voltage does not change (much). Thus, Vout also will not change, even if a load connected to Vout draws current (current that would have otherwise passed through the Zener diode).
If the small amount of inherent Zener diode voltage variability is unacceptable, Vout can feed into an LDO or DC/DC power supply, which will provide improved output regulation.
Constant Power Loss
The Zener diode introduces a non-obvious drawback common to all transformerless power supplies: constant power consumption regardless of load. The current passing through Z1 can go one of two places: through the Zener diode or through the load connected to Vout. However, the total average current will always match the current through Z1. For a transformerless supply that can source up to 30mA:
- If the load connected to Vout draws very little current (or none at all), then all unused current (up to 30mA) flows through Z2 which dissipates power in the Zener diode.
- If the load connected to Vout draws most of the 30mA, then the power dissipation of the Zener is lower while the power dissipation of the load is higher.
Output (Hold-up) Capacitance
A rectified sinusoidal AC input voltage (as shown in Figure 1) has periods of time where the instantaneous Vin has a smaller magnitude than the DC output (Zener) voltage. To prop up the DC output voltage during these periods, a capacitor is added to Vout. This capacitor allows Vout to “ride through” the periods of small instantaneous AC voltage.
Input Impedance
Z1 is usually implemented as one of two options. A very simple low cost Z1 is a resistor; a more efficient option is a capacitor. The size of the Z1 resistor or capacitor and the Zener voltage together determine how much total output current will be available.
Blocking Diode Placement
There are two places where blocking diode(s) for rectification can be placed: before the Zener diode and after the Zener diode. In general, placing a blocking diode after the Zener ("post-Zener") will prevent the (admittedly small) reverse current flow from the output capacitor through the Zener. The output capacitor generates reverse current flow through the Zener only during portions of the waveform where Vin is less than the output capacitor voltage. Inclusion of a Post-Zener diode results in a tradeoff that the output voltage will typically be a diode drop (0.7V) less than the Zener voltage.
Full Wave Rectified Circuits:
For full-wave rectification to be effective, the rectification must be performed before the Zener diode (that is, the full bridge rectifier must be between the AC source and the Zener diode). This is because the Zener diode will only generate the Zener voltage output whenever a reverse voltage is applied to it. Full wave rectification ensures that Vin is positive, which allows the Zener voltage to be generated. If full wave rectification were added after the Zener diode (between the Zener and the output capacitor), then the negative portion of the AC waveform would simply result in forward conduction through the Zener, which does not generate a useful output voltage. Therefore, for full-wave rectification, blocking diodes must always be present before the Zener diode (“pre-Zener”). An optional blocking diode may still be placed after the Zener. However, in full wave rectified circuits this is typically not done; the opportunity for the output capacitor to discharge through the Zener in reverse occurs so infrequently that that the leakage is not a concern.
Half Wave Rectified Circuits:
If half wave rectification is used, a single post-Zener blocking diode may be used with no pre-Zener blocking diode. Post-Zener blocking diodes provide greater benefit in a half wave rectified circuit because (as shown in the waveform of Figure 1) at least 50% of the time, Vin sits at 0V, which is less than the output voltage, giving plenty of opportunity for capacitor leakage through the Zener. The leakage is even more evident in low voltage Zener diodes (<6V typically) because their current-voltage curve tends to be “softer”—that is, the Zener may start conducting current well before the Zener voltage is reached. In many cases, however, the leakage currents even without a Post-Zener blocking diode is usually low enough that it is not a concern.
Resistive And Capacitive Input Impedance
As mentioned previously, there are both resistive and capacitive options for the input impedance selection. The purpose of the input impedance is to provide a large voltage drop from Vin to Vout. For resistive input impedance, it should come as no surprise that this large voltage drop generates substantial power loss when compared against capacitive input impedance.
Table 1 provides a comprehensive list of the basic transformerless power supply configurations, along with the tradeoffs encountered by each configuration. The basic configurations are constructed from the following options:
- Capacitive or Resistive Input Impedance
- Full Wave or Half Wave Rectification
- Pre-Zener or Post-Zener Rectification
Note that full-wave configurations with post-Zener rectification are entirely excluded from the list because it is not possible to generate full-wave rectification after the Zener has effectively passed only a half-wave rectified output. The Capacitive Half-Wave Rectified configuration with Pre-Zener Rectification is shown for illustrative purposes only as it does not generate output voltage.
Resistive Transformerless AC Supply
The lowest-cost, physically-smallest component we can usefor input impedance Z1 is a resistor, which we will refer to as R1 or Rin. All load and Zener diode current flows through R1. Large peak voltages (as high as Vin,peak - Vout) will exist across R1, so it must be rated to handle high voltage. The combination of high voltage and current flow (equal to the total load + Zener current) results in significant power lost as heat in resistor R1. A resistive transformerless is typically sized to deliver relatively small amounts of current (a few mA) so as to limit the heat generated by resistor R1.
The power dissipated by resistor R1 for a full-wave rectifier is the RMS voltage across R1 squared, divided by the resistance R1. We can approximate the RMS voltage across R1 from the RMS of the input voltage minus the DC output voltage.
Ploss,R1 = (Vin,rms - Vout)² / R1
All current that flows through R1 has the potential to become output current. It is only when the Zener diode conducts current that the current is lost as heat in the Zener. Otherwise, the output capacitor effectively stores R1 current, allowing loads to draw current from the output capacitor as needed. Based on capacitor charge (amp-second) balance principles, we know that the output capacitor will provide as many amp-seconds to the load as it receives from R1. Thus, the average current through R1 represents the maximum average output current that the resistive transformerless power supply can generate. The word “maximum” is used here because less current can always be delivered to the output (down to 0A), in which case the Zener consumes the unused current.
R1 will pass an average current based on the average voltage applied across it. For a full wave rectified AC input, this means the output current will be based on the average of the absolute value of the AC waveform. Looking back at Figure 1 for the full wave output, we see that simply calculating the average AC voltage for the positive ½ of one sine wave period will provide the average AC voltage for the entire waveform.
For a half wave rectified AC input instead, this means the output current will be based half of the average of the positive part of the AC waveform (because the other half of the waveform will be 0V). This effectively reduces the average output current by half compared to the full wave rectified configuration.
Advanced calculation: As a technical matter, the voltage across the resistor in both cases is reduced by the output voltage. For small output voltages (less than 10V) it is reasonable to use Vpk-Vzener whereas for large output voltages >10V, simulation is preferred because the voltage across R1 begins looking less and less like a normal sinusoid.
Device tolerances and safety margin (typically reducing power ratings of components by 50%) should be considered when sizing components for power loss. Further guidance for worst-case power sizing is provided in the notes of the accompanying spreadsheet.
Capacitive Transformerless AC Supply
Typically a capacitive transformerless supply is used to delivery larger amounts of current (tens of milliamps) than resistive transformerless supplies, specifically because the improved efficiency enables it without additional cooling concerns.
If we use a capacitor (C1 or Cin) for input impedance Z1, the efficiency improves because we are no longer “losing to heat” the current that was flowing through it as R1. Rather, the amp-seconds are stored in the capacitance of C1 without loss, and then C1 is discharged without loss. Instead of the output current being related to Vpk / R1, it is related to C1 (dV(t) / dt.
Input Resistance for Inrush Limiting
Despite the naturally reduced losses for the capacitive transformerless supply, resistive loss is still added back in intentionally. As for why: if a capacitor is connected directly to the mains at an instant when the AC voltage is at a peak value, the large voltage will rapidly charge up the capacitor, which appears (albeit briefly) to be a short circuit.
The high current that charges up the capacitance can potentially exceed upstream circuit breaker current limits, causing them to trip and generate localized power outages. Additionally, the high inrush current can cause undesirable arcing at the moment the device is plugged in. To prevent high inrush currents, a small resistance is usually placed in series with C1.
The resistance should be small enough that it does not generate much heat, but should be large enough that it
limits short circuit currents to acceptable levels. Common 120VAC household circuit breakers in the US are typically rated for 15A. If 5A is an acceptable maximum short circuit current, then the resistance should be no less than R1 = Vpk/5A = 170V/5A = 34Ω. For the inrush current to remain less than 0.4A, then R1 = 470Ω will do nicely.
All current that flows through R1 has the potential to become output current. It is only when the Zener diode conducts current that the current is lost as heat in the Zener. Otherwise, the output capacitor effectively stores R1 current, allowing loads to draw current from the output capacitor as needed. Based on capacitor charge (amp-second) balance principles, we know that the output capacitor will provide as many amp-seconds to the load as it receives from R1. Thus, the average current through R1 represents the maximum average output current that the resistive transformerless power supply can generate. The word “maximum” is used here because less current can always be delivered to the output (down to 0A), in which case the Zener consumes the unused current.
R1 will pass an average current based on the average voltage applied across it. For a full wave rectified AC input, this means the output current will be based on the average of the absolute value of the AC waveform. Looking back at Figure 1 for the full wave output, we see that simply calculating the average AC voltage for the positive ½ of one sine wave period will provide the average AC voltage for the entire waveform.
For a half wave rectified AC input instead, this means the output current will be based half of the average of the positive part of the AC waveform (because the other half of the waveform will be 0V). This effectively reduces the average output current by half compared to the full wave rectified configuration.
Advanced calculation: As a technical matter, the voltage across the resistor in both cases is reduced by the output voltage. For small output voltages (less than 10V) it is reasonable to use Vpk-Vzener whereas for large output voltages >10V, simulation is preferred because the voltage across R1 begins looking less and less like a normal sinusoid.
Device tolerances and safety margin (typically reducing power ratings of components by 50%) should be considered when sizing components for power loss. Further guidance for worst-case power sizing is provided in the notes of the accompanying spreadsheet.
Capacitive Transformerless AC Supply
Typically a capacitive transformerless supply is used to delivery larger amounts of current (tens of milliamps) than resistive transformerless supplies, specifically because the improved efficiency enables it without additional cooling concerns.
If we use a capacitor (C1 or Cin) for input impedance Z1, the efficiency improves because we are no longer “losing to heat” the current that was flowing through it as R1. Rather, the amp-seconds are stored in the capacitance of C1 without loss, and then C1 is discharged without loss. Instead of the output current being related to Vpk / R1, it is related to C1 (dV(t) / dt.
Input Resistance for Inrush Limiting
Despite the naturally reduced losses for the capacitive transformerless supply, resistive loss is still added back in intentionally. As for why: if a capacitor is connected directly to the mains at an instant when the AC voltage is at a peak value, the large voltage will rapidly charge up the capacitor, which appears (albeit briefly) to be a short circuit.
The high current that charges up the capacitance can potentially exceed upstream circuit breaker current limits, causing them to trip and generate localized power outages. Additionally, the high inrush current can cause undesirable arcing at the moment the device is plugged in. To prevent high inrush currents, a small resistance is usually placed in series with C1.
The resistance should be small enough that it does not generate much heat, but should be large enough that it
limits short circuit currents to acceptable levels. Common 120VAC household circuit breakers in the US are typically rated for 15A. If 5A is an acceptable maximum short circuit current, then the resistance should be no less than R1 = Vpk/5A = 170V/5A = 34Ω. For the inrush current to remain less than 0.4A, then R1 = 470Ω will do nicely.
