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SMPS design using TinySwitch®-III technology has a very wide input range of 18VDC - 265VAC
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Switching power supplies are common with a lot of applications such as consumer products, appliances and industrial applications. They are more efficient and have many advantages compared to non-switching power supplies. |
Most of all they are easy to design for a wide range input ac voltage (85VAC – 265VAC) which allows a worldwide use of the supplied equipment using one power supply only.
Furthermore the switching technology makes it possible to expand the input voltage range from 24VDC up to 265VAC. This flexible power supply could therefore be used on the industrial market. In control cabinets you either have a 85VAC – 265VAC supply voltage for the installed equipment or a 24VDC bus system. Using just one single power supply for both systems would reduce the overall costs significantly.
This article is meant to describe an isolated 3W power supply using TinySwitch ®-III technology from Power Integrations with a very wide input voltage range of 18VDC – 265VAC including a concise description of a hardware prototype with both measurement results and design guidelines.
TinySwitch ®-III Description
TinySwitch ®-III combines a high voltage power MOSFET switch with a power supply controller in one device. Unlike conventional PWM (pulse width modulator) controllers, TinySwitch ®-III uses a simple ON/OFF control to regulate the output voltage. This makes the regulation circuit very simple because no compensation is required.
The highly integrated device contains a 700V power MOSFET switch, a switch controller, an oscillator and many support/ protection functions such as auto-restart, thermal shutdown, under-voltage shutdown and cycle-by-cycle current limiting. The integrated current limit state machine of TinySwitch ®-III ensures an audio-noise-free operation of the power supply as it changes its current limit to lower value when the load is decreasing. In effect there is a rise in switching frequency so that it won’t be lower than 20kHz at any load condition.
Figure 1 shows a power supply using TinySwitch ®-III 280PN. The floating constant current source is necessary to power up the supply at very low input voltages under 50VDC. For universal input voltage (85VAC – 265VAC) you won’t need an extra circuit.
The ultra wide input voltage transformer was designed with the PI Expert™ Software from Power Integrations using its Excel based version PI Xls Designer especially.
Primary-side circuit operation
The ac input Voltage is rectified and filtered by D2, C3 and C4, creating a high voltage dc source. L1 completes a simple pi-filter (C3, C4 and L1) which is sufficient for passing the EMI-Test. The patented Primary E-Shield TM from Power Integrations in combination with the integrated frequency jittering of TinySwitch ®-IIIallows the use of this kind of simple filtering.
The RCD-Clamp (R2, C5, R3 and D6) limits the peak drain voltage which is caused by leakage inductance. Resistor R3 allows the use of a slow, low cost rectifier diode by limiting the reverse current through D6 when U1 turns on. This will increase the efficiency of the power supply and reduces the heating in the clamp itself.
U1 employs an ON/OFF control to regulate the output in response to the feedback signal received by the EN/UV pin. During normal operation switching is disabled when a current greater than 95µA is pulled from the EN/UV pin.
The bypass capacitor C6 is charged by the floating constant current source in order to provide TinySwitch ®-III with sufficient current to maintain operation under all conditions.
The transformer has 35 primary and 14 secondary turns which amounts to a transfer ratio of 1:2.5 and a primary inductance of about 330µH. This is quite low for a Flyback converter at that power level. Otherwise it would not be possible to get the maximum output power of 3W at low dc voltage input. At full load and maximum load respectively TinySwitch ®-III is running in fully continuous mode whereas at high voltage input the power supply operates in discontinuous mode. TinySwitch ®-III can be driven in both modes without having a bad impact on the output voltage regulation due to its simple voltage mode regulation scheme.
Secondary-side circuit operation
Output rectification is provided by D9. The low ESR capacitor C10 achieves a minimum output voltage ripple. Output voltage is regulated by a simple 2% Zener diode feedback (VR3). R9 is used to ensure that the Zener diode is biased at its test current. The Zener regulation method provides sufficient accuracy. This is possible because TinySwitch ®-III limits the dynamic range of the optocoupler LED current allowing the Zener diode to operate at near constant bias current.
Startup and Floating Constant Current Source
Under usual conditions, at 85VAC – 265VAC Input Voltage, the capacitor C6 is charged via internal current source after applying the mains voltage. When the voltage in the Bypass (BP) pin reaches 5.8V the device starts switching. TinySwitch ®-IIIdoes not need a bias winding at all to supply itself. For maximum efficiency and lowest no load losses an extra bias winding on the transformer is recommended. With Bias Winding no load consumption is less than 50mW, without Bias Winding less than 250mW is possible.
The minimum specified Drain-Source voltage of TinySwitch ®-IIIis 50VDC according to the datasheet. Basically there is no problem to start up the power supply with a lower voltage than 50VDC except for the effect on the startup and auto-restart behavior of the switcher. The startup circuit (Q1, Q2, VR4, D11, D12 R7, R5 and R8) is necessary to ensure a safe startup of the Power Supply under worst-case conditions. This floating constant current source provides a constant current of about 400µA to the BP pin of TinySwitch ®-III over the complete input voltage range.
R8 together with VR4 determines the constant current value:
Q1 delivers the constant current at lower input voltages and Q2 at higher input voltages. Figure2 shows the simulation results of the current flow through Q1 and Q2. Up to an input voltage of about 50VDC the constant current is supplied by Q1. From 50VDC on the current through Q1 will decrease and the current through Q2 is increasing in a linear way. At the maximum input voltage of 375VDC the constant current is provided primarily by Q2.
R5 is used to limit the input current of the whole circuit at maximum input voltage. Resistor R7 defines where the current curves of I CQ1 and I CQ2 intersect.
Figure 2 – Simulation of Constant Current over Input Voltage
At low input voltage the voltage difference between V Q2base to V Q2collector equals zero. With a rising input voltage the difference grows bigger while Q2 starts conducting and the current I Q2CE increases. At the same time the voltage V Q1base decreases leading to a decline in the current flowing through Q1.
The non-linear current rise comes from the non-linear behavior of the Zener diode VR4. At about 60VDC input voltage the Zener Voltage is reached.
Circuit Performance
A prototype has been built to measure the performance of the ultra wide input power supply.
Figures 3a and 3b show the efficiency over low DC or rather high AC input voltage under full load conditions. The maximum efficiency is 75% and the lowest efficiency was measured at 18VDC input voltage and is about 63%.
Figure 3c shows the no load consumption of the power supply at high AC line input voltage. The maximum measured no load consumption is 161mW at maximum input voltage. The startup circuit is connected via D12 directly to the mains voltage not to the DC bus. This reduces the no load consumption at high line voltage input as the average voltage applied to the startup circuit is lower. The no load losses are mainly caused by switching losses. The no load consumption at low DC Input voltage is negligible.
Line regulation behavior is shown in Figure 3d. The measured line regulation tolerance was lower than 0,5% over the complete input voltage range (18DC – 265VAC) at full load conditions.
Conclusions & Summary
It is possible to extend the output rating by using TOPSwitch ®-GX or PeakSwitch TM from Power Integrations, an integrated off-line switcher with a higher output power capability. The PWM controlled TOPSwitch ®-GX offers more features like under- and over-voltage shutdown, line feed forward and different packages for different power levels. The TOPSwitch ®-GXfamily can be used for a maximum output power of 290W for single input voltage (230VAC ± 15%).
For a 24VAC input voltage the single rectifier should be replaced by a bridge rectifier while the input capacitors C3 and C4 must be changed to a higher capacitance. This is necessary in order to keep the Drain-Source voltage over a minimum of 18VDC.
The demand for ultra wide input voltage power supplies is constantly growing. Linear power supplies using a 50Hz transformer, that are not qualified for a wide input voltage range, will soon be widely replaced by switching solutions. Power supplies based on Power Integrations designs can beat the price of material cost of linear power supplies while meeting those specifications that a linear transformer based power supply cannot cost-effectively support.
For more information on the full range of Power Integrations products available from Anglia please call +44 (0)1945 474747 or email info@anglia.com
