Switched Mode Power Supply Examples
In this section we show three different levels of modeling switched mode power supplies. In the fourth section various implementations per converter type are available for download
Open LoopWe start with a constant dutycycle, ideal semiconductor models and with the parasitic ESR of the output capacitance and ESL of the inductor.
Closed LoopIn the next level we include the feedback by using a generic control block, mostly voltage mode control and current mode control. Also a gate driver is included.
Detailed semiconductor modelsFinally in the third level, we use a control IC to implement the feedback and we add parasitic components that are responsible for ringing around the switching node.
Various implementationsFor each type of converter various implementations are shown. The levels are mixed and different aspects of modeling are demonstrated
In the open loop simulations, the duty cycle of the switching Mosfets is constant. Only a few components are required and the open loop is merely to justify component size and voltage, current and power levels.
The basic circuit elements are found in the
The scope and ground symbol are selected from the bottom button-bar. It is suggested to start a new simulation example by first adding the ground symbol on your schematic, so the ground node is included.
Notice that the gate of the Mosfet and IGBT are system nodes, they can directly be connected to the system control blocks as well as the constant duty-cycle blocks.
Since we are simulating open loop, there is no need for control, only a block giving a constant duty-cycle. Notice that he output of these blocks are system nodes, so they directly connect to the gates of the Mosfet and IGBT selected from Components/Circuit/Semiconductor.
There are various blocks for single ended, dual ended and synchronous PWM, as well special blocks for interleaved PWM and phase shifted control signals as required for Full Bridge and Dual Active Bridges. The constant PWM blocks are found in Components/Library/PowerConverters/SMPS/PWM.
The coupled inductors are found in Components/Library/PowerConverters/SMPS/CoupledInductors/Ideal.
Connecting to Mosfet with system node gate
The feedback is implemented using the generic control blocks for SMPS. Here we can choose if we want to use the ideal circuit mosfet model with system control node or if we are going to use the mosfet models with an electric gate connection. If you quickly want update the open loop examples, the blocks from the section Components/Library/PowerConverters/SMPS/Control/CurrentMode and Components/Library/PowerConverters/SMPS/Control/VoltageMode as can be seen on the right side of the figure below. Als the opamps(with electric circuit connections) and comparator with system output Components/Library/PowerConverters/SMPS/Control/Opamp connect directly to the system node gate of the Mosfet.
Connecting to Mosfet with electric node gate
If you would like to connect the system output to a more detailed mosfet model or a mosfet model with electric circuit node gate connection, the drivers from the section Components/Library/PowerConverters/SMPS/Driver should be used.
Generic control blocks
In the Closed Loop example section we will only use the generic control blocks and gate driver blocks that have an electrical circuit node output and therefore (via an external gating resistor) connect to the electrical gate node of the mosfet model.
The generic models are especially designed to make fast closed loop simulations since they have a build-in oscillator. Therefore the simulation step size dt can still be relative large. The single ended model is for the Buck, Boost, BuckBoost and flyback converters. The double ended model is for Push-Pull converters. The GateDriverSyn block can be used for a synchronous buck and the more general GateDriver block is intended for the Half and Full Bridge as well as the Dual Active Bridge.
The generic blocks are on the Components/Library/PowerConverters/SMPS/Control/Generic section.
Control components such as opamps, comparators, opto-couplers and a voltage regulator can be found in the Components/Library/PowerConverters/SMPS/Control/Opamp, Components/Library/PowerConverters/SMPS/Control/OptoCoupler and Components/Library/PowerConverters/SMPS/Control/TL431 section.
Mosfets and BJTs
Semiconductor switches like mosfets, and bipolar transistors can be found in the semiconductor Components/Library/PowerConverters/Semiconductors/Mosfet and Components/Library/PowerConverters/Semiconductors/BJT section. The MosfetLevel0 model only has the gate capacitance modeled, the MosfetLevel1 also includes two parasitic inductors with value LLead at the Drain and Source connections as well as the output capacitance Cds.
Detailed semiconductor models
When the closed control loop is being verified by simulation, the next step is to add more detail in the simulation. This is done in multiple ways
- Add series resistance for every inductor and capacitor(Although you could do this with generic simulations too).
- Change the generic control into a control IC or build the control from analog and digital circuit models. Adding bandwidth and maximum current sourcing and sinking as well limited voltage levels of control IC's are influencing the closed control loop.
- Add loop inductance around the Mosfet.
- Change the ideal semiconductor models with a more detailed model, for example, replace the nMosfetLevel0 with the VDMOS model
- Add a thermal model or at last specify the worst case temperature for the semiconductors.
- Add a more detailed model for the gate driver and the power supply for the gate driver, for example, when the control IC is fed from the aux winding of the coupled inductors or transformer.
- Build a custom magnetic circuit for the transformer or coupled inductors using the magnetics library.
- Replace any digital logic circuit with more detailed CMOS IC models.
For the Mosfet the MosfetLevel0, MosfetLevel1 and VDMOS are pin compatible. The VDMOS has a thermal node on the right side and the parameters can be given via the model node. The BJT is more or less ideal except for its miller capacitance which influences the bandwidth of the device. The JFET is only a static device, but capacitance can be added externally.
The gate drivers have internally delay and losses and therefore give a better result when it comes to switching times of the mosfets. Also the charge pump can be added for the high-side driver, to simulate the procedure for charging the bootstrap capacitor.
To get a more realistic view of the circuit, breadboard components can be used. Internally they are equal to the models in semiconductor/IC.