Despite the huge advantages that switching power supplies have in terms of end-user convenience — small size, light weight, universal input voltage operation, and significant energy saving benefits — manufacturers often select linear ac adapters to meet the tight cost goals for a product.
However, a new high-voltage IC will challenge the ac adapter market to move from a linear to switching topology. This IC, designated LinkSwitch (LNK501), enables a switching power supply solution that rivals the linear transformer in simplicity and cost while retaining the energy efficiency, universal input voltage range, size, and weight advantages of traditional switching power supplies.
Recent moves in energy conservation have made switching power supplies more attractive, although cost remains prohibitive. New standby energy requirements stipulate as little as 0.3W input power under no-load conditions, which until now have increased the complexity and cost of both linear and switching adapters.
Using a new topology, LinkSwitch requires as few as 14 components for a complete power supply. It also replaces all linear transformer and RCC- (ringing choke or self-oscillating converters) based chargers and adapters up to 3W for single and universal input voltages. It's suitable for any application that requires up to a 3W linear transformer.
As well as outperforming alternative solutions in terms of efficiency and component count, a LinkSwitch supply is simpler to design and less expensive than alternative switching solutions, such as a discrete PWM controller and MOSFET and RCC supplies. For example, it has 30 fewer components than a typical RCC solution — a 68% reduction.
The IC's EcoSmart™ feature allows designs to meet both the U.S. Presidential 1W initiative for standby power covering any electronic device and the more stringent European 0.3W requirement for input power when the output of a charger is unloaded. In comparison, a typical linear supply will consume more than 1W — even when disconnected from the load. Therefore, a charger with the new IC can pay for itself with reduced energy costs within the first few months of operation.
Supplies employing the new IC are also suited to replace linear “bricks” in cordless phones and answering machines. Recently, the EPA has introduced its Energy Star guidelines for these devices. The ever-growing demand for energy efficiency in other applications, previously using linear bricks, is expected to further accelerate the adoption of these supplies as a cost-effective alternative.
Integration Is Key
To meet the tough cost requirements, an IC solution must integrate as many functions as possible — yet be cost effective. The new device achieves this by monolithically integrating a 700V power MOSFET and controller into a three terminal power IC housed in a low cost 8-pin DIP package. Integrated functions include startup, PWM control, 42-kHz oscillator, current limit, auto-restart and thermal shutdown for fault protection, and EcoSmart low standby power mode.
With a handful of external components, a designer can produce a 3W CV/CC (constant voltage/constant current) charger. This output voltage characteristic is significantly better controlled over line and load than a linear adapter. The additional feature of a current limited output, without any secondary current sense components, makes it ideal for battery charging applications. What makes the device unique however, is the level of integration plus a topology and control scheme that allows dramatic simplification.
Fig. 1 shows the adapter schematic and Fig. 2, on page 24, is the output characteristic for a universal input, 2.75W output power supply using only 14 components (assuming integrated bridge rectifier). In this example, the output voltage at the peak power point is 5.5V and the constant current limit is 500mA. These limits are fully adjustable during development. The design meets both CISPR22B/EN55022B and FCC B conducted EMI limits with good margin and does not use a Y capacitor, making it ideal for designs that require low leakage currents (<5µA). Under no-load conditions, the supply consumes <250mW at 230Vac and <200mW at 115Vac input.
Unlike traditional switching supplies, where the switching element is placed on the low side of the transformer, the LNK501 is on the high side. By referencing the IC to the rectified dc input, you can derive all feedback information for an approximate constant voltage and constant current operation from the primary side clamp circuit. This removes the need for a secondary side current sense circuit and associated components (saving about eight components). Removing the secondary current sense resistor improves efficiency, reducing the secondary loss by approximately 0.5W for a 500mA, 5.5V output and increasing efficiency by >10%.
After applying power to the circuit, a switched high-voltage current source connected internally between the Drain and Control pins charges Control pin capacitor C3. Once charged, this stored energy powers the IC. The IC begins normal operation and MOSFET switching transfers energy to the secondary where D6 and C5 rectify and filter it.
By minimizing IC power consumption, the primary-side leakage inductance clamp energy (D5 and C4 in Fig. 1) also powers the LNK501 during normal operation. This energy, which is normally wasted, is thus put to good use. In addition to providing the IC supply current, the clamp is the source of all feedback information. The voltage developed across C4 is an approximate representation of the output voltage transformed through the transformer turns ratio. Resistor R1 converts this reflected voltage or VOR (voltage, output reflected) into a current (IC) and then feeds it into the Control pin. LinkSwitch has three operating modes determined by the Control pin current, as illustrated by the Control pin characteristics shown in Fig. 3, on page 24.
Feedback and Control
The LNK501 operates as a discontinuous flyback with voltage mode control for the CV portion and current limit operation for the CC portion of the output characteristic. Blue lines in Fig. 3 indicate the region of current limit control. During startup, as the output voltage and therefore the VOR increases, the control current IC increases. Accordingly, the internal current limit increases, reaching 100% when IC equals IDCT. This provides an inherent soft-start characteristic. Once the output voltage reaches regulation, control of the output transitions to constant voltage. However, if the output load increases beyond the peak power point and the output voltage falls, the reduced Control pin current lowers the internal current limit, providing an approximately constant current output characteristic.
Besides improving energy efficiency, this simplifies the circuit design by removing the need for an auxiliary or bias winding on the transformer and associated components (saving another three components).
Red lines in Fig. 3 indicate the region of duty cycle control. Under normal load conditions when IC exceeds IDCS, the duty cycle is controlled, providing an approximately constant output voltage. For light-load/no-load conditions, when the duty cycle drops below 3%, the switching frequency reduces to cut energy consumption.
To provide a CV/CC output characteristic, the flyback transformer is always discontinuous; for each switching cycle all the energy is delivered to the load during the MOSFET off time. The power, P, transferred by in the transformer during discontinuous operation is:
P = ½×L×I2×f
L = Primary inductance
I = Peak primary current
f = Switching frequency.
The discontinuous mode delivers the same power regardless of input voltage, so the LNK501 provides up to 3W from either 85Vac to 265Vac or 185Vac to 230Vac input voltage ranges. The internal 42 kHz switching frequency allows the delivery of 3W using a compact, low-cost EE13 core size while still allowing a very simple, input-conducted EMI filter.
Since the current limit and frequency determine the peak power or CV/CC transition point in the output characteristic, the parameter of current limit squared times switching frequency (I2LIM×fOSC) is specified in the datasheet. This, together with the primary inductance, defines the variation in the CV/CC transition point. Using low-cost transformer construction, a primary inductance tolerance of ±10% is typical, giving an overall variation in the peak power point, including temperature, of ±20% in high-volume manufacturing.
Output voltage regulation depends on how well the voltage across C4 tracks the reflected output voltage. Although transformer leakage inductance affects the tracking, standard transformer construction techniques in the power supply provide a better output load regulation than a linear transformer. This is acceptable in most low power applications. Typical tolerance of ±10% at the peak power point includes 85Vac to 265Vac line voltage variations, device variation, and temperature effects. For applications requiring tighter voltage tolerance, add an optocoupler and secondary reference to provide ±5% across the whole load, line, and temperature range.
When a fault condition, such as an output short circuit or open loop, prevents flow of current into the Control pin, capacitor C3 discharges and the LNK501 enters auto-restart mode. Here, the device periodically restarts the power supply to restore normal power supply operation after removal of the fault.
The LNK501 is available in a design accelerator kit containing prototype adapters and samples, a full prototype engineering report, and an application note (DAK-16).
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