This article explores a variety of approaches to the design of a 2 Watt isolated DC/DC power supply. It includes information about an Isolated Switching Regulator with Integrated Feedback. This information may be used by power supply designers who want to improve output voltage accuracy over a wide operating temperature range without adding the complexity of designing compensation networks to stabilize the control loop. The function of an isolated DC /DC power supply is to provide a stable DC voltage at the secondary side. A well designed closed loop power supply is required to provide good load regulation and transient response, which are fundamental to a stable DC output voltage. Many isolated closed loop designs use a controller on the primary side where the power switches are located and must obtain the isolated output voltage information from the secondary side. Using a secondary side controller would make it easier to detect the secondary side output voltage, but it requires complexity in adding a secondary side startup voltage circuit and providing isolation for driving the primary side switches. The simplicity of primary side controllers is why most designs use them, and a focus of this article will be on the different ways of detecting the isolated output voltage information in isolated DC/DC converters and the limitations of these methods.
The traditional isolated DC/DC power supply design uses an optocoupler for isolated feedback and a shunt regulator as a secondary side error amplifier and reference. The primary side controller topology with optocoupler feedback is a flyback converter, as shown in Figure 1. The flyback circuit has the simplicity of using just one switch on the primary side, and a rectifying diode on the secondary side. The shunt regulator provides the reference voltage that is compared to the divided down output voltage by the internal error amplifier. The output is fed to the optocoupler LED driver circuit. The error amplifier needs a compensation network to stabilize the voltage loop, which requires engineering time to develop. The optocoupler LED current is provided by the shunt regulator output biased by a series resistor. The amount of current needed is based on the optocoupler current transfer ratio (CTR) characteristics as described in its datasheet. The CTR characteristics are the ratio of output transistor current to input LED current, which are not linear, and vary from part to part. Optocouplers typically have a two to one uncertainty in initial CTR, and will have 50% less CTR after years of use or service in high temperature environments found in high power and high density supplies. Often seen as an inexpensive isolator for use in a DC/DC power supply, the optocoupler variation in CTR limits the voltage feedback performance and the effective operating temperature range.
Figure 1. Primary Side Controller with Optocoupler Feedback
Primary Sensing Regulator without Optocoupler
To avoid the use of optocoupler feedback, one can use switching regulators that rely on primary current sensing. These devices use a closed loop flyback architecture as shown in Figure 2, and they rely on the relationship of primary current and transformer turns ratio to control the output voltage. To indirectly sense the output voltage from the primary side, the flyback voltage is sampled by an error amplifier in the primary side controller which uses this feedback to control the output voltage.
VFLYBACK ≈ (VOUT + VDIODE)*(NPRI/NSEC) (1)
The output voltage is not directly measured and the flyback voltage equation (1) shows the dependence on the secondary side diode voltage. Any change in diode voltage will contribute to output voltage variations. The problem with this approach is that the output diode forward voltage can vary with load current and temperature, producing errors in the output voltage.
Figure 2. Primary Sensing Regulator without Optocoupler
Isolated Switching Regulator with Integrated Feedback
The Isolated Switching Regulator with Integrated Feedback approach uses precision circuits and a digital isolator to directly sense and isolate the output voltage. This eliminates poor output voltage feedback performance caused by optocoupler CTR variations. Directly sensing the output voltage produces stable output voltage performance, unlike primary sensing regulators that are dependent on diode voltages that vary with load current and temperature. In the block diagram of Figure 3, an integrated error amplifier senses the voltage of the output divider and the secondary side controller uses a digital isolator transformer to send the pulse width modulated (PWM) feedback signal to the primary side. The primary side logic has gate drivers to control the X1 and X2 switches, which in turn controls the energy sent through the power transformer to the secondary side.
The Isolated Switching Regulator replaces the optocoupler, shunt regulator and compensation network in the traditional flyback circuit, with the secondary side controller circuits. The startup circuits for the secondary side controller are internal to the primary control logic, which contributes to greater ease of use. The precise integration of these functions eliminates the time and effort of designing external startup circuits and compensation networks. In addition, the Isolated Switching Regulator has two internal push-pull switches on the primary side to drive the transformer, which reduces the external components to a minimum and the push-pull topology helps improve efficiency.
Figure 3. Isolated Switching Regulator with Integrated Feedback
The benefits of the Isolated Switching Regulator with Integrated Feedback can be seen over the Primary Sensing Regulator in their performance curves. Figure 4 Load Regulation Characteristics, shows that the Isolated Switching Regulator has a near constant output voltage with about 1mV change from 50mA to 500mA output current. For comparison, the Primary Sensing Regulator output changes by more than 50mV over the same load conditions. The next performance curve is Figure 5, the temperature performance of the output voltage. The Isolated Switching Regulator has only a 15mV change in the 5.0V output over a wide temperature range of -40°C to 105°C. The Primary Sensing Regulator has a poor performance of over 120mV change in output voltage over this same temperature range. These variations in the Primary Sensing Regulator output voltage are due to the changes in the forward diode voltage caused by load current through the diode and the temperature dependence of the forward diode voltage. The last performance curve is the efficiency curve in Figure 6. Again, the Isolated Switching Regulator comes out ahead with over 80% efficiency in the light load region of 100mA, where the Primary Sensing Regulator has a poorly designed light load efficiency of 60%.
Figure 4. Load Regulation of Isolated Switching Regulator Vs Primary Sensing Regulator
Figure 5. Temperature Performance of Isolated Switching Regulator Vs Primary Sensing Regulator
Figure 6. Efficiency of Isolated Switching Regulator Vs Primary Sensing Regulator
The Isolated Switching Regulator with Integrated Feedback uses precision circuits and a digital isolator to directly sense and isolate the output voltage. Directly sensing the output voltage produces stable output voltage performance, unlike primary sensing regulators that are dependent on diode voltages that vary with load current and temperature. The digital isolator has precision circuits without a dependence on CTR variations found with optocouplers, producing an output voltage with a high accuracy across a wide temperature range. In addition, this approach integrates a secondary side controller and error amplifier with an internal compensation network, which significantly reduces the size, complexity and design time.
source: http://www.powerpulse.net/powerViews.php?pv_id=82&page=2
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