Bias power made easy
Many offline-powered systems today need a low-voltage bias supply to power its control circuitry and intelligence. In some cases, bias supply outputs are not required to be isolated from the AC mains, but are used to power a system microcontroller, LED display, drive relays or AC switches. Some examples include home automation, e-metering, standby power supplies in TVs, home appliances, and many others.
End equipments have become more power-hungry with time as more and more “smarts” and functionalities are being added. Electricity meters, for instance, have transformed from simple metrology with just energy measurement and mechanical displays, to smart e-meters that incorporate radio frequency (RF) communication such as Wi-Fi, ZigBee® and/or power line communication (PLC), LCD displays, AC disconnect relays, and so on. Consequently, their AC/DC power supply requirements have transformed from a single output rail with a few mA to multiple rails with hundreds of mA.
There are a few different topology options to consider for bias-supply designs. The classic 60-Hz step-down transformer and AC capacitive-drop solutions are both well-known and robust solutions. However, they fall short when it comes to efficiency, size and standby power performance. Similarly, an isolated-flyback switch-mode power supply would be far too complex and expensive to design for this need.
Equipment power consumption regulations[1] have further driven the need for high efficiency bias-supply designs. Bias supplies with very good light-load efficiency are required to enable more active system functions in standby mode, while keeping the total end equipment consumption to a minimum. In this article we discuss the key requirements and challenges in designing off-line, non-isolated bias supplies and how high-voltage IC technology can help simplify such designs.
Let’s first discuss some basic requirements and key attributes when it comes to designing off-line bias supplies. Some features and system benefits are summarized in Table 1. The priority and importance of each feature listed is highly specific to application and equipment.
Table 1: Typical bias-supply requirements and system benefits
The classic linear regulator, also known as an low-dropout (LDO) solution (Figure 1) presents an easy-to-use solution. The front-end, implemented with a high-voltage depletion-mode MOSFET, handles the majority of voltage drop and power dissipation, allowing the downstream stage to employ standard low-voltage, low-power LDO regulators. The regulated bias output VOUT can be easily modified for other output voltages by changing the LDO rating.
Albeit simple, one of the key limitations with this design is power dissipation in Q1, which can be calculated using equation (1), where VD can be approximated to the RMS value of the input voltage and IZ is the sum of the bias currents for the zener D1 and U1. A typical 230V AC input design with 10V/50 mA output results in approximately 10W power dissipation in Q1 alone. This yields a very poor conversion efficiency and large heat-sinking to dissipate this power.
Figure 1: Linear drop bias supply
To improve conversion efficiency, the front-end regulator can be replaced with a less dissipative capacitive-drop power supply, which is essentially a voltage divider across the input (Figure 2). In order to meet apparent power regulations, the RMS input current IIN must be limited. This is achieved by the series RC impedance, as shown in equation (2). Here VHFRMS is the RMS voltage of a half-wave AC sine wave, and XC1 is the reactance of C1. Hence, each capacitive drop supply design is good only for a narrow range of AC line voltage and frequency.
Note that the cap drop output voltage V2 remains constant, so long as its output current I2 is less than or equal to IIN. The power capability of this approach can be slightly increased by improving downstream efficiency with a DC/DC switching converter. Besides power limitations, a fundamental disadvantage with this approach is that the input power consumption is independent of the load. Hence, as power levels increase, this approach does not meet the stringent standby power regulation requirements.
Figure 2: Capacitor divider bias supply
Switch-mode power supplies effectively can help address the efficiency and standby power limitations discussed earlier. An approach widely used today is a high-side buck converter implemented using a pulse-width modulation (PWM) controller IC in voltage-mode feedback control (Figure 3). The switching power device Q1 can be a high-voltage N-channel or P-channel MOSFET with N-channel devices. These are usually preferred due to their wide availability and performance metrics (RDSON and gate charge) versus an equivalent die size P-channel MOSFET. However, they require a floating gate drive circuit when using a ground-referenced low-voltage controller.
The controller start-up circuitry using high-voltage resistors R3 and R4 is required to bias the controller at start-up. After the output voltage is regulated, the controller bias current can be derived directly from the output using D2. However, if the converter output voltage VOUT is lower than the required bias voltage for the controller Vcc (for example 3.3V output), an additional auxiliary winding is required on the buck inductor to boost the voltage which further complicates the design.
Note that R3 and R4 remain connected to the high-voltage input and present a fixed standby power sink for the converter. Additional control circuitry is required to differentially sense the power MOSFET current and protect it during over-load and short-circuit fault conditions.
Finally, this converter can be required to operate at very narrow duty cycles depending on the bias-supply design. A 400-V input to 5-V output design operating at 100 kHz switching frequency requires a MOSFET on time of ~125 nsec. This can represent significant challenges when we consider device-switching speed limitations, high-side driver propagation delays, and leading- edge blanking for accurate MOSFET current-sensing and protection.
Figure 3: Off-line buck converter with a PWM controller IC
High-voltage IC technology can address these design challenges, enabling designers to reduce their component count and design-cycle times significantly. A high-voltage switcher such as the UCC28880 [2] (Figure 4) integrates a 700-V power MOSFET, a high-voltage current source for start-up, current-sensing, control circuitry and fault protection in a compact SO-7 package. The switcher incorporates a soft-start feature for controlled power stage start-up to minimize stress on the power stage components.
With smart power management, its bias current consumption is reduced to less than 100 uA. This means that the switcher can be self-biased from the AC input during normal operation and standby without significantly impacting overall system power consumption. Output voltage regulation is achieved by replicating the output voltage on capacitor C4 during the off time of the buck converter. The switcher implements smart over-load protection by increasing the converter off time during over-load and short-circuit conditions to prevent inductor current runaway and MOSFET overstress.
It also incorporates an over-temperature monitoring circuit to protect itself and the system during an over-temperature fault condition. The switcher can be used in various application topologies with direct or isolated feedback, and in low-side or high-side buck configurations, depending on the application bias needs. Efficiency and output regulation accuracy for a universal AC-input bias supply with 13V/100 mA output is shown in Figure 5. The UCC28880 varies the converter switching frequency across load to enable a flat efficiency curve with best-in-class average efficiency. Please refer to [3] for full test data.
Figure 4: Non-isolated buck converter in high-side configuration
Figure 5: Efficiency and output voltage accuracy for a 1.3 W bias supply
Summary
Off-line bias-supply design specifications and performance requirements vary widely, based on the application and end equipment. However, most system designers are seeking low component count solutions that offer ease of use and simple scaling to reduce overall system design cycle time. In this article, we discussed how high-voltage IC technology [4] and how smart power management can greatly simplify non-isolated AC/DC bias power supply designs. Provided as an example is the UCC28880 700-V switcher which enables robust, low-cost, transformer-less designs using standard off-the-shelf components to deliver best-in-class efficiency and standby power performance.
References
1) European Commission’s Ecodesign Directive for energy-related products (ErP) Lot 6, Tier 2 requirement for household and office equipment effective in 2013 limits total system standby power consumption to less than 0.5 W.
2) UCC28880 datasheet
3) UCC28880 high-side buck evaluation module
4) More information about high-voltage AC/DC solutions
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