Switching Chip Tames Power Peaks

Jul 1, 2006 12:00 PM
By Stefan Bäurle, Principal Engineer, Power Integrations, San Jose, Calif.

A variable frequency switching regulator for ac-dc power supplies drives peak loads from a design optimized for lower continuous loads to enable compliance with efficiency mandates.

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Products such as inkjet printers, data-storage devices, audio amplifiers and dc motor drives require power supplies that can deliver up to threefold peak-to-continuous load ratios. The conventional approach to designing such power supplies has typically involved sizing all of the power components to deliver the peak power levels as though they were continuous operating conditions. That approach impacts the cost and size of the power supply, while also making it difficult to satisfy emerging energy-efficiency standards for no-load and standby power consumption.

In response to these challenges, a power-conversion IC with peak power management technology has been developed. A detailed circuit example demonstrates how this IC may be applied in power-supply designs to simultaneously meet application requirements and energy-efficiency regulations while managing overall design costs.

The PeakSwitch IC is designed for applications with high peak-to-continuous power requirement ratios up to 300%. Fig. 1 shows a typical peak power application employing the PeakSwitch.

The PeakSwitch incorporates a 700-V power MOSFET, an oscillator with frequency jittering for low EMI, a high-voltage switched current source for startup and a current limit in a single monolithic device. In addition, a variety of protection features including auto-restart, line undervoltage sense and hysteretic thermal shutdown have been added.

The simple on/off control scheme with four discrete current limit levels offers various advantages over traditional PWM-controlled power supplies.^[1] On/off control responds to a feedback signal and enables or disables primary-side switching in order to transfer energy appropriate to the load conditions at the output of the power supply. (Reference 2 at the end of the article will direct you to a detailed description of the on/off operating principles.) Besides the fact that loop compensation is not needed, it also allows the PeakSwitch to operate at very high switching frequencies of up to 277 kHz during peak loads. Because with on/off control a switching cycle is only initiated when energy transfer is required, the effective average switching frequency during lighter load conditions will be much lower.

Fig. 2 plots the average switching frequency of a 32-W continuous, 81-W peak power supply based on the PeakSwitch at five distinct load conditions. During “super peak” power (81 W), the effective switching frequency is very high at 240 kHz. It reduces to 130 kHz at full continuous load (32 W) and to 18 kHz during sleep mode (P_IN = 3 W). In standby mode (P_IN = 1 W), the frequency is reduced to 3 kHz, and with no load attached, it drops to only 0.3 kHz. The very high effective frequency under peak load conditions allows the transformer core size to be minimized. With PeakSwitch, the core size may be chosen for continuous load conditions to meet the thermal requirements, because the increase of effective switching frequency at peak loads will not increase the core flux density. On the other hand, traditional PWM-controlled power supplies typically run at a fixed frequency of only 60 kHz to 100 kHz over the entire load range up to the maximum peak load. Therefore, the transformer core size must be selected for peak load conditions to avoid saturation when the primary current is increased to satisfy the peak load requirement.

A useful new feature is the integrated, programmable smart ac-line sensing with fast ac reset. If case regulation is lost — for instance, due to an output short circuit, open control loop or brownout — the device stops switching after 30 ms. After that period, PeakSwitch continuously monitors the status of the ac input voltage via the optional smart ac sense circuit shown in Fig. 1. If regulation is lost but the ac input is still present, a fault is assumed and the device latches off. For resetting the latch, the power supply has to be unplugged from the ac inlet and, a few seconds later, connected again. Once the IC detects this sequence, the latch is reset and a restart attempt is initiated when the ac input is restored. This feature provides a low-cost latching shutdown fault protection with a fast ac reset and few additional components. If regulation is lost and the ac input is not connected or is at unusually low levels, switching also ceases but the supply will not be latched off. Once the ac input returns to normal levels, switching is resumed.

PeakSwitch also offers various protection features. The tightly toleranced thermal shutdown can protect the entire power supply in case of a thermal overload condition. A large hysteresis provides auto-recovery without the need to add a separate reset circuit. The cycle-by-cycle current limit protects the integrated power MOSFET from excessive drain currents. During high-line operation, the IC decreases the current limit by 10% to compensate for the normal overshoot caused by the current-limit propagation delay. Therefore, the available high-line overload power is also reduced.

Another new and useful feature in the PeakSwitch is the integrated on-time extension function. When PeakSwitch detects abnormally low input-voltage conditions, it adapts by extending the maximum MOSFET switch on-time (defined by the maximum duty ratio specification in the data sheet). The on-time is extended for as long as it takes the primary current to reach the device current limit. Therefore, the energy transferred to the load is significantly increased. This effectively increases the available peak power during very low line conditions. The holdup time with a given bulk capacitor size may be increased, which in turn may reduce overall system cost.

Fig. 3 illustrates on-time extension at work during a standard line sag test as defined by IEC61000-4-11 (120 Vac, 60% dip, 10 cycles at full load). Unlike the system without this feature, the peak drain current reaches the device current limit even during the abnormal low line condition and, thus, keeps the output in regulation throughout the 10 reduced line cycles. The power supply without this feature loses regulation very quickly and the output drops after only a few line cycles. A more detailed description of all integrated functions and features in the PeakSwitch can be found in the data sheet.^[2]

Design Example

Fig. 4 depicts a typical peak power application example using PeakSwitch. It delivers 32 W of continuous and 81 W of peak power. Because of the high switching-frequency operation described previously, the design employs a small EE-25 core size for the transformer. Alternative designs using traditional control concepts switch at much lower frequencies and, therefore, must use larger and more expensive core sizes such as the EER-28 or larger.

Resistors R5 and R6 set the line undervoltage lockout threshold, which prevents startup at unsafe line voltages and output glitches during power down or brownout. Resistor R16 provides a small amount of bias current to the EN/UV pin to retain the undervoltage lockout function during brownout conditions. The actual input-voltage threshold is governed by the values of these resistors, the EN/UV pin line undervoltage threshold current (I_LUV = 25 µA), the EN/UV pin voltage (V_EN/UV = 2 V with I_EN/UV = 25 µA) and the bypass pin voltage (V_BP = 5.8 V) as follows:

By rearranging Eq. 1 and with the shown resistor values plus the typical data sheet values mentioned previously, the startup threshold in this example is set at Because D5 and C7 are peak charging the ac input, this corresponds to a minimum ac startup voltage of 78 Vac.

Diode D5 and capacitor C7 form the smart ac sense and fast-reset function as explained previously. The actual line voltage (V_LINE) at which the power supply has latched off, the size of C7 and the EN/UV pin I_LUV set the time required to reset the latch once the input voltage has been removed:

It is obvious that the worst-case reset time appears at the maximum input voltage: V_LINE = 265 Vac. For the component values shown in Fig. 4, the worst-case reset time equals 1.1 sec. The load connected to the power supply is protected in case of an overload fault by the simple current-sense circuit formed around transistor Q1 and resistor R9 and the integrated latch function. The low-pass filter R10 and C13 add a minimum trip delay (t_TRIP) before the SCR Q2 is fired. The overcurrent trip point (I_OCP) and t_TRIP are governed by Eqs. 3 and 4 as follows:

Assuming a typical base-emitter voltage of the latching overcurrent protection is triggered if a current equal or greater than I_OCP = 2 A is drawn from the load for t_TRIP = 210 ms. The latch function of PeakSwitch significantly reduces the size and, hence, the cost of the SCR and the output rectifier D8, as overload current only flows for 30 ms before the supply latches off. Therefore, the SCR should be selected with a current rating above the continuous output current of the supply. An open-loop fault condition (for example, defect optocoupler) is sensed via the Zener VR3, which then also fires the SCR Q2. Subsequently, the power supply latches off until the ac input is disconnected and reintroduced again.

Thanks to on/off control, the design example has an excellent efficiency performance. The efficiency is constant across the entire load range (Fig. 5). The table summarizes the power-supply performance at prominent load conditions defined by various agencies around the globe.

The active-on efficiency that is achieved easily meets the minimum value of (0.49 + 0.09 × ln(32)) × 100%) = 80.2% as specified by the California Energy Commission (CEC) and others.

For printer applications, the introduction of a new operating condition — sleep mode — is currently being discussed by Energy Star. In this mode, the printer appears to be inactive to the user; however, it is able to start printing instantaneously with the push of a button. The power consumption in this mode is limited to a target of 3 W, yet the printer control circuitry has to be completely energized.

The PeakSwitch-based power supply can deliver 2.3 W of output power with only 3 W of input. In standby mode, with the input power being limited to 1 W per U.S. Executive Order 13221, the power-supply example delivers 0.7 W to the load.

A more detailed description of the design example including schematic, layout, bill of materials, transformer specification and performance details is available from the company.^[3]

References

1. TNY263-296 TinySwitch-II Family data sheet, Power Integrations, April 2005.

2. PKS603-606 PeakSwitch Family data sheet, Power Integrations, March 2006.

3. Engineering Prototype Report for EP-93, 32 W/81 Wpk Supply Using PeakSwitch (PKS606Y), Power Integrations, March 2006.

Table. Efficiency performance summary for power-supply circuit in Fig. 4.

Average Active-On Efficiency
VIN (Vac)	25% load	50% load	75% load	100% load	Average
115	81%	81.6%	82.3%	82.4%	81.8%
230	80.5%	81.5%	82.9%	83.1%	82%
Sleep Mode
VIN (Vac)	POUT (W)	PIN (W)	Efficiency
115	2.34	3	78%
230	2.30	3	76.7%
Standby Mode
VIN (Vac)	POUT (W)	PIN (W)	Efficiency
115	0.72	1	72%
230	0.70	1	70%
No-Load Input Power
VIN (Vac)	85	115	230	265
PIN (W)	0.091	0.102	0.158	0.183