Power Electronics

High Voltage ICs Simplify Off-Line Supplies

Following up its earlier success with single chip, off-line supplies, Power Integrations has come up with the TinySwitch II®, an upgraded version of their previous TinySwitch family. Like the original TinySwitch, the IC integrates a 700V power MOSFET, oscillator, high-voltage switched current source, current limit, and thermal shutdown into a monolithic IC. Fig. 1 shows a typical TinySwitch II supply.

You derive start-up and operating power directly from the voltage on the drain pin, this eliminates the need for a bias winding and associated circuitry. In addition, the TinySwitch-II incorporates auto-restart, line undervoltage sense, and frequency jittering. Its design minimizes audio frequency noise with a simple On/Off control scheme using ordinary taped/varnished transformer construction. The fully integrated auto-restart circuit safely limits output power during fault conditions, such as output short circuit or open loop, reducing component count and secondary feedback circuitry cost.

An optional line sense resistor externally programs a line undervoltage threshold, which eliminates power down glitches caused by the slow discharge of input storage capacitors present in applications such as standby supplies. Jittering its 132 kHz operating frequency reduces quasi-peak and average EMI, minimizing filtering cost.

Overtemp Protection

Internal thermal shutdown circuitry senses the die temperature. The threshold is typically set at 135°C with 70°C hysteresis. If the die temperature rises above this threshold the chip disables the power MOSFET, and it remains disabled until the die temperature falls by 70°C, at which point it is re-enabled.

A current limit circuit senses the power MOSFET's current. If it exceeds the internal threshold, the chip turns the power MOSFET off for the remainder of that cycle. The current limit state machine reduces the current limit threshold by discrete amounts under medium and light loads.

Leading edge blanking inhibits the current limit comparator for a short time after the power MOSFET turns on. This blanking time prevents current spikes caused by capacitance and secondary-side rectifier reverse recovery time from causing premature termination of the switching pulse.

In the event of an output overload, output short circuit, or an open loop condition, TinySwitch-II enters into auto-restart operation. An internal counter clocked by the oscillator gets reset every time the EN/UV pin goes low. If the EN/UV pin isn't pulled low for 50 ms, the power MOSFET switching will disable for 850 ms (except in the case of line undervoltage condition in which case it's disabled until the condition is removed). Auto-restart alternately enables and disables the power MOSFET's switching until removal of the fault condition.

You can monitor the dc line voltage by connecting an external resistor from the dc line to the EN/UV pin. During power-up or disabled switching of the power MOSFET in auto-restart, the current into the EN/UV pin must exceed 50µA to initiate power MOSFET switching. During power-up, the chip holds the BYPASS pin to 4.8V, while line undervoltage condition exists. The BYPASS pin rises from 4.8V to 5.8V when the line undervoltage condition goes away. The auto-restart counter stops when you disable the MOSFET switching and a line undervoltage exists. This stretches the disable time beyond its normal 850 ms until the line undervoltage condition ends.

These devices operate in the current limit mode. When enabled, the oscillator turns the power MOSFET on at the beginning of each cycle. The MOSFET turns off when the current ramps up to the current limit, or when it reaches the dc max limit. The highest current limit level and frequency are constant, so the power delivered to the load is proportional to the primary inductance of the transformer and peak primary current squared. Therefore, designing the supply involves calculating the primary inductance of the transformer for the maximum output power required. By choosing the appropriate TinySwitch-II for the power level, the current in the calculated inductance will ramp up to current limit before reaching the dc max limit.

Pricing for the TinySwitch II family in 1,000 piece quantities ranges from $0.74 each for the TNY264P, a 5W part in a plastic 8-pin DIP, to $1.09 each for the 23W rated TNY268G, housed in an 8-pin SMD package.

New Off-line Supply IC

A new member of the single-chip, off-line supply world is STMicroelectronics' L6590 IC. It simplifies 15W off-line power supply designs by integrating a 700V power MOSFET switch, plus all of the control circuits needed to build a flyback, boost, or forward converter. The device operates over the universal input voltage range of 85Vac to 265Vac and also with dc inputs, making it a flexible solution for consumer applications. Fig. 3 shows the device in a flyback circuit.

An on-chip 65 kHz oscillator eliminates the need for external oscillator components and a non-dissipative internal startup circuit guarantees high efficiency at light load and also further reduces the external parts count. Standby operating mode ensures high efficiency at light loads (Pin<200mW at Po = 0W) meeting the tightest regulations concerning standby operation. A brownout function prevents spurious turn-on at power down and protects against input line undervoltage. Also, an on-chip error amplifier improves primary regulation, avoiding the cost of an external optocoupler and op amp feedback network.

The input bulk capacitor must be large enough to produce a relatively low amount of ripple. At minimum line voltage, the value of this capacitor determines the absolute minimum of the input voltage applied to the converter. Don't exceed the maximum duty cycle and the maximum peak current allowed by the IC at this voltage. Large values of this capacitor result in lower peak and rms current through the MOSFET and less duty cycle range to achieve regulation. Smaller values of this capacitor cause the opposite condition. A good compromise is a capacitor value that causes the peak-to-peak ripple amplitude to be 25% to 30% of the peak line voltage. If the system requires holdup capability, a larger capacitance will be necessary.

Typical applications of L6590 include wall plug power supplies, ac-dc adapters, auxiliary supplies for CRTs, LCD monitors, desktop PCs and servers, fax machines and laser printers.

Three package versions are available. The L6590D is a 16-lead Small-Outline package (SO-16W) that provides both the brownout and error amplifier connections. Types L6590N and L6590AN are in 8-lead DIP packages and exclude either the error amplifier (L6590AN) or the brownout function (L6590N). Design support for the L6590 includes an evaluation package, the EVAL6590N, which contains samples, an evaluation board and PC software, which simulates the application function. This software assists in the selection of external components and simulates overall application performance.

The L6590 is fabricated using proprietary bipolar-CMOS-DMOS (BCD)-off-line technology, which allows the integration of 700V power devices and control circuits on the same chip. This technology is used in a variety of applications including off-line power supplies and energy-saving electronic lamp ballasts. BCD-off-line is a high voltage implementation technology family applied in a range of smart power applications ranging from simple motor drivers to complex smart power subsystems with embedded microcontrollers. Pricing of the L6590 is $0.75 in 25K quantities.

Single-Cell Charge Pumps

Ideal for use with low-power microcontrollers, the Texas Instruments' TPS6030x family of single-cell charge pumps provides a regulated 3.0V or 3.3V output at up to 20 mA from a single NiMH, NiCd or alkaline battery. These charge pumps convert input voltages ranging from 0.9V to 1.8V at either a 3x or 4x rate, depending on the input level. This flexible conversion rate compensates for the lower voltage level in a partially discharged battery, enabling more efficient power usage throughout the discharge life of the battery. These new switched-cap converters also enable use of smaller external capacitors, allowing them to reduce board space and component costs.

The TPS6030x charge pumps are voltage quadruplers that provide a regulated 3.3V or 3.0V output from a 0.9V to 1.8V input. They deliver a maximum load current of 20mA. Designed for space critical battery powered applications, the complete converter requires only five external capacitors and enables the design to use low-cost, small-sized, 1 µF ceramic capacitors. The TPS6030x circuits consist of an oscillator, a voltage reference, an internal resistive feedback circuit, an error amplifier, two charge pump stages with MOSFET switches, a shutdown/start-up circuit, and a control circuit, as shown in Fig. 4.

Driving EN low disables the converter. This disables all internal circuits, reducing input current to only 0.05µA. Leakage current drawn from the output pins OUT1 and OUT2 is a maximum of 1µA. The device exits shutdown once EN is set high. The typical no-load, start-up time is 400 µs. When the device is disabled, the load is isolated from the input. This is an important feature in battery-operated products because it extends the battery shelf life.

Setting EN from logic low to high enables the device. CP1 (2x doubler) enters a dc start-up mode during which the capacitor on OUT1 charges up to about VIN. After that, it starts switching to boost the voltage further up to about two times VIN. CP2 (4x doubler) then follows and charges up the capacitor on OUT2 to about the voltage on OUT1. After that, it also starts switching and boosts the voltage to its nominal value. EN must not exceed the highest voltage applied to the device.

Devices include a newly developed LinSkip mode that enables the power supplies to achieve an efficiency of up to 90% over a wide load range. This new technology also allows designers to use 1 µF capacitors, and provides a typical output voltage ripple of 30mVpp at an output current of 20mA. This is a reduction of 40% at twice the output current when compared with similar devices. Under light loads, the charge pump automatically switches from low-noise, constant-frequency linear regulation to a power-saving pulse-skip mode.

Load isolation disables current flow to the system during shutdown, providing longer shelf and standby times for the battery. Operating efficiency is increased with a low quiescent current of less than 35µA during operation and less than 500nA during shutdown.

The power-good output is an open-drain output on the TPS60300 and TPS60301 or a push-pull output on the TPS60302 and TPS60303. The PG-output pulls low when the output of OUT2 is out of regulation. When the output rises to within 98% of regulation, the power-good output goes active high. Power-good pulls low in shutdown. In normal operation, an external pullup resistor with the TPS60300 and TPS60301 typically connects the PG pin to VOUT. The resistor should be in the 100 kΩ to 1 MΩ range. If the PG output is not used, it should remain unconnected. Output current at PG (TPS60302, TPS60303) will reduce maximum output current at OUT2.

The devices come in a 10-pin MSOP package. Suggested resale pricing is $1.25 in quantities of 1000 units. An Evaluation Module (EVM) is available for the TPS60302.

Power Integrations, San Jose, Calif.
CIRCLE 348 on Reader Service Card

STMicroelectronics, Inc, Lexington, Mass.
CIRCLE 350 on Reader Service Card

Texas Instruments, Dallas
CIRCLE 351 on Reader Service Card

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