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--- university:courses:electronics:switched-cap-power-supplies [07 Apr 2018 00:04] – Formatting Mark Thoren
+++ university:courses:electronics:switched-cap-power-supplies [25 Jun 2020 22:07] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
-====== Activity: Power Conversion: Switched Capacitor Methods ======
+====== Activity: Switched Capacitor Power Supplies ======
 ===== Objective: =====
@@ Line 11: / Line 11: @@
 Linear regulators have widespread application and are well deserving of their own activity, but there are some things that a linear regulator just can't do:
-"Boost" a lower voltage to a higher voltage
+  * "Boost" a lower voltage to a higher voltage
-"Invert" a positive voltage to a negative voltage
+  * "Invert" a positive voltage to a negative voltage
-Reduce a high voltage to a lower voltage any more efficiently than could be achieved with a power resistor in series with the supply, at an equivalent output current.
+  * Reduce a high voltage to a lower voltage any more efficiently than could be achieved with a power resistor in series with the supply, at an equivalent output current.
 This is where other techniques must be employed. One such technique is "switched-capacitor" voltage conversion, and conceptually involves little more than capacitors, switches, and some form of control to the switches.
@@ Line 28: / Line 29: @@
 V USB power supply\\
 Voltmeter (optional, can use M2K in Voltmeter mode.)\\
+LTspice files for this activity: {{ :university:courses:electronics:sw_cap:switch_cap_ltspice_files.zip |}}\\
-====== Warmup: Human-switched capacitor power conversion ======
+====== Activity 0: Human-switched capacitor power conversion ======
 The introduction alluded to being able to hold a capacitor with stored energy in your hands. Let's do a quick experiment and make a human-switched-capacitor voltage inverter.
-Say you have a 9V battery, and you want to make a split +/-9V supply. Hold a 330uF cap to a 9V battery (OBSERVE PROPER POLARITY!), then connect cap (+) to battery (-), measuring from battery (-) to cap (-).
+Say you have a 9V battery, and you want to make a split +/-9V supply. Charge the 220uF cap from the parts kit by holding it to the terminals of a 9V battery (OBSERVE PROPER POLARITY!), then connect cap (+) to battery (-), measuring from battery (-) to cap (-).
+{{ :university:courses:electronics:sw_cap:human_inverter_phase_0.jpg?400 |}}
+{{ :university:courses:electronics:sw_cap:human_inverter_phase_1.jpg?400 |}}
-=re-take photos=
+<WRAP centeralign> Figure 1. Human switched-cap inverter </WRAP>
 This "flips" the positive voltage across the capacitor below ground, and you now have a split supply... until the capacitor discharges, either through your circuit, or through internal leakage.
@@ Line 41: / Line 45: @@
 Similarly, if you have a 9V battery need a higher voltage, say 18V, you can charge the capacitor again, then connect capacitor (-) to battery (+). This "stacks" the positive voltage across the capacitor on top of the battery's own 9V, producing 18V... until the capacitor discharges, either through your circuit, or through internal leakage.
+{{ :university:courses:electronics:sw_cap:human_doubler_phase_0.jpg?400 |}}
+{{ :university:courses:electronics:sw_cap:human_doubler_phase_1.jpg?400 |}}
+<WRAP centeralign> Figure 2. Human switched-cap doubler</WRAP>
 If you do this fast enough, your circuit won't notice the switching and will see a continuous, steady voltage, but it would quickly get tiring for the human switcher.
-=re-take photos=
 With a basic idea of switched capacitors, let's proceed to something more practical.
-====== Switched Capacitor Voltage Inverter ======
+====== Activity 1: Switched Capacitor Voltage Inverter ======
+===== Theory and Simulation =====
 Some op-amp circuits can operate on a single supply, with the op-amp negative supply pin connected to ground. However there are applications that benefit from the use of a "split" supply with voltages of opposite polarity referred to circuit ground, for example, with the op-amp's supply pins connected to positive 5V and negative 5V. Such a supply can be created by using two 9-V batteries, an LM7805 positive regulator, and an LM7905 negative regulator. But this extra battery is an inconvenience - when is the last time you saw a product that required TWO 9V batteries?
 This is where a switched-capacitor inverter can come in handy - A single battery can now provide both a positive supply (direct connection to the positive terminal), and a negative supply (through an inversion circuit.) Before testing real circuits, let's get a feel for how the circuit works by runnign an idealized simulation. This simulation essentially automates the procedure from Experiment 0.
-The LTspice file is included here: (link to ltspice file)
+Open the //switch_cap_inverter.asc// schematic from the zip file in LTspice.
 The key element in this simulation is a switch:
 {{ :university:courses:electronics:sw_cap:switch.png?200 |}}
+<WRAP centeralign> Figure 3. LTspice switch</WRAP>
 which is assigned a value of **my_sw**. Any switch in the schematic with value **my_sw** will have properties defined by the spice directive:
@@ Line 76: / Line 82: @@
 The switches are controlled by two pulse generators, with outputs labeled **clk** and **clk_bar**.
-{{ :university:courses:electronics:sw_cap:clocks.png?250 |}}
+{{ :university:courses:electronics:sw_cap:clocks.png?400 |}}
+<WRAP centeralign> Figure 4. Clock Sources</WRAP>
 Right-clicking will bring up the parameter windows:
 {{ :university:courses:electronics:sw_cap:clk_param_window_phase0.png?400 |}}
+<WRAP centeralign> Figure 5. clk Parameters</WRAP>
 {{ :university:courses:electronics:sw_cap:clk_param_window_phase1.png?400 |}}
+<WRAP centeralign> Figure 6. clk_bar Parameters</WRAP>
-Each source outputs 3 microsecond pulses with a 10 microsecond period. The only difference is that V3 is delayed by 5us - this produces a "non-overlapping clock", which allows switches to be alternately turned on and off, and never on at the same instant. Running the simulation, and probing q and q_bar, shows what's going on:
+Each source outputs 3 microsecond pulses with a 10 microsecond period. The only difference is that V3 is delayed by 5us - this produces a "non-overlapping clock", which allows switches to be alternately turned on and off, and never on at the same instant. Running the simulation, and probing clk and clk_bar, shows what's going on:
 {{ :university:courses:electronics:sw_cap:nonoverlapping_clocks.png?400 |}}
+<WRAP centeralign> Figure 7. Non-overlapping Clock Waveforms</WRAP>
 The last step in making the idealized simulation is to put the pieces together, to do what human hands did in experiment zero. When **clk** is asserted (at 1V, closing S1 and S2), the capacitor is charged to +5V:
 {{ :university:courses:electronics:sw_cap:switch_cap_inverter_phase_0.png?600 |}}
+<WRAP centeralign> Figure 8. Inverter Phase 0</WRAP>
 When **clk_bar** is asserted, the left terminal of C1 that was charged to 5V is then grounded (to zero volts), which drives the right terminal negative:
 {{ :university:courses:electronics:sw_cap:switch_cap_inverter_phase_1.png?600 |}}
+<WRAP centeralign> Figure 9. Inverter Phase 1</WRAP>
 Run the simulation, and probe **vout**, **clk**, and **clk_bar**.
 {{ :university:courses:electronics:sw_cap:switch_cap_inverter_waveforms.png?400 |}}
+<WRAP centeralign> Figure 10. Inverter Startup Waveforms</WRAP>
 Notice that the output voltage does not immediatly reach its final value, but takes several "steps." This is because while C1 is initially charged to 5V, C2 is initially discharged. Thus the charges in each capacitor are:
@@ Line 107: / Line 120: @@
 When S3 and S4 close, C1 and C2 are placed in parallel, so the 5uc is then divided among two 1uF capacitors, resulting in a voltage of -2.5V. Subsequent charge / discharge cycles drive the output voltage closer to its final value of nearly -5V. The 1k load resistor prevents the output from ever reaching exactly -5V, but if the switching is fast enough, it can come close.
+===== Circuit Construction and Testing =====
+With the simulation understood, let's move on to actual components. Open the //LT1054_inverter.asc// schematic from the zip file in LTspice. Construct the circuit on a breadboard, following the LTspice schematic.
-With the simulation understood, let's move on to actual components. Construct the LT1054 inverter circuit on a breadboard, following the LTspice schematic:
+{{ :university:courses:electronics:sw_cap:lt1054_inverter_schematic.png?600 |}}
+<WRAP centeralign> Figure 11. Inverter Breadboard circuit</WRAP>
-{{ :university:courses:electronics:sw_cap:1054_inverter.zip |}}
 Build the following breadboard circuit for the voltage inverter.
-{{ :university:courses:electronics:sw_cap:lt1054_inverter_breadboard.png?600 |}}
+{{ :university:courses:electronics:sw_cap:lt1054_inverter_breadboard.png |}}
+<WRAP centeralign> Figure 12. Inverter Breadboard circuit</WRAP>
 The circuit can also be soldered on a "Perma Proto" solderable breadboard from Adafruit, which matches the layout of typical solderless breadboards.
-{{ :university:courses:electronics:sw_cap:lt1054_inverter.jpg?400 |}}
+{{ :university:courses:electronics:sw_cap:lt1054_inverter.jpg?600 |}}
+<WRAP centeralign> Figure 13. Inverter Circuit Soldered on PermaProto board</WRAP>
 Connect a voltmeter (or M2K in voltmeter mode) between circuit ground and the OUT pin of the LT1054, and Apply 5V to the IN pin. The voltmeter should read close to -5V.
@@ Line 130: / Line 146: @@
 Set Scopy to Oscilloscope mode, with the following settings:
-  * Timebase: 200us/div
+  * Timebase: 250us/div
   * CH1, CH2: 1V/div
   * Triggering: Ch1, -1V, Falling Edge, Single Shot mode.
@@ Line 136: / Line 152: @@
 Momentarily short LT1054 pin 1 (FB) to ground. This disables the LT1054. Release FB; this allows the LT1054 to operate again, and produces a "clean" turn-on transient. You should see a waveform similar to the figure below:
-{{ :university:courses:electronics:sw_cap:lt1054_inverter_turn_on_scopy.png?600 |}}
+{{ :university:courses:electronics:sw_cap:lt1054_inverter_turn_on_scopy.png |}}
+<WRAP centeralign> Figure 14. LT1054 Inverter Startup Scopy Measurement</WRAP>
 Run the LTspice simulation, and probe the corresponding nodes. You should see results similar to the figure below:
 {{ :university:courses:electronics:sw_cap:lt1054_inverter_turn_on_ltspice2.png?600 |}}
+<WRAP centeralign> Figure 15. LT1054 Inverter Startup LTspice Simulation</WRAP>
 This shows reasonable correlation between the simulation and actual measurements. This is a good thing, but it's always important to keep in mind that:
@@ Line 152: / Line 170: @@
 {{ :university:courses:electronics:sw_cap:lt1054_inverter_turn_on_ltspice_w_cap_current.png?600 |}}
+<WRAP centeralign> Figure 16. LT1054 Inverter Startup Simulation w/ Capacitor Current</WRAP>
 If you built this circuit up on a PermaProto board, you can put it in a box and use it with your next project that requires a split (positive and negative) power supply.
 //Be sure to read the pin description for Vout in the LT1054 datasheet!//
+===== Questions: =====
+It was mentioned above that "if the switching is fast enough, the output can come close (to -5V)." How close? Activity 19 derives an expression for the equivalent resistance of a switched-capacitor circuit, and the same analysis can be applied to the switched-capacitor inverter. The LTspice simulation of the (close to) ideal inverter switches a 1uF capacitor at 100kHz. What is the equivalent resistance at the output? This resistance represents a theoretical limit to the power supply's load regulation performance. Try increasing the size of the output capacitor to "smooth out" the ripple due to individual switching events, and decrease the value of the load resistor such that the output voltage is decreased by 10% (to -4.5V.)
+The LT1054 has additional losses due to voltage drops across the switching transistors; this is discussed in detail in the datasheet.
-====== Charge Pump Voltage Doubler ======
+====== Activity 2: Charge Pump Voltage Doubler ======
-Another useful power conversion function is producing a high voltage from a lower voltage, which was demonstrated in the second half of Experiment 0. The LT1054 switches are not configured in a way that will perform this function directly, but we can use the LT1054 to drive a "charge pump" to double the input voltage. Once again, let's start with a close to ideal simulation to illustrate the idea. The LTspice file is included here: switch_cap_pump_doubler.asc
+===== Theory and Simulation =====
+Another useful power conversion function is producing a high voltage from a lower voltage, which was demonstrated in the second half of Experiment 0. The LT1054 switches are not configured in a way that will perform this function directly, but we can use the LT1054 to drive a "charge pump" to double the input voltage. Once again, let's start with a close to ideal simulation to illustrate the idea. Open the //switch_cap_pump_doubler.asc// schematic from the zip file in LTspice.
 This circuit borrows the nonoverlapping clocks from the inverter simulation. The two states are easy to visualize:
@@ Line 164: / Line 187: @@
 **clk** asserts, turning S4 on, pulling the lower terminal of C1 to ground. D2 conducts, charging C1 to Vusb (minus a diode drop.)
-{{ :university:courses:electronics:sw_cap:switch_cap_pump_doubler_phase_0.png?600 |}}
+{{ :university:courses:electronics:sw_cap:switch_cap_pump_doubler_phase_0.png?500 |}}
+<WRAP centeralign> Figure 17. Doubler Phase 0</WRAP>
 **clk_bar** asserts, turning on S1. The lower terminal of C1 is driven to Vusb, and the Pump node is driven to Vusb plus another Vusb (minus a diode drop.)
-{{ :university:courses:electronics:sw_cap:switch_cap_pump_doubler_phase_1.png?600 |}}
+{{ :university:courses:electronics:sw_cap:switch_cap_pump_doubler_phase_1.png?500 |}}
+<WRAP centeralign> Figure 18. Doubler Phase 1</WRAP>
 The result is that Vout is "pumped" to 2X Vusb, minus two diode drops. Probing Vout and Pump confirms this:
 {{ :university:courses:electronics:sw_cap:switch_cap_pump_doubler_waveforms.png?600 |}}
+<WRAP centeralign> Figure 19. Doubler Startup LTspice Simulation</WRAP>
-As with the inverter circuit, the output takes several clock cycles to reach its final value due to charge sharing. With the simulation understood, let's move on to actual components. Construct the LT1054 doubler circuit, following the LTspice schematic:
+As with the inverter circuit, the output takes several clock cycles to reach its final value due to charge sharing.
+===== Circuit Construction and Testing =====
-The LTspice file is included here: (insert file)
+With the simulation understood, let's move on to actual components. Construct the LT1054 doubler circuit, following the LTspice schematic //LT1054_doubler.asc// from the zip file.
+{{ :university:courses:electronics:sw_cap:lt1054_doubler_schematic.png?600 |}}
+<WRAP centeralign> Figure 20. Doubler Schematic</WRAP>
 Build the following breadboard circuit for the voltage inverter.
-{{ :university:courses:electronics:sw_cap:lt1054_doubler_breadboard.png?600 |}}
+{{ :university:courses:electronics:sw_cap:lt1054_doubler_breadboard.png |}}
+<WRAP centeralign> Figure 21. Doubler Breadboard circuit</WRAP>
 The circuit can also be soldered on a "Perma Proto" solderable breadboard from Adafruit, which matches the layout of typical solderless breadboards.
 {{ :university:courses:electronics:sw_cap:lt1054_boost.jpg?600 |}}
+<WRAP centeralign> Figure 22. Doubler Circuit Soldered on PermaProto board</WRAP>
-Connect a voltmeter (or M2K in voltmeter mode) between circuit ground and the OUT pin of the LT1054, and Apply 5V to the IN pin. The voltmeter should read close to -5V.
+Connect a voltmeter (or M2K in voltmeter mode) between circuit ground and the OUT pin of the LT1054, and Apply 5V to the IN pin. The voltmeter should read close to +8.6V.
 Next, let's take a look at what the circuit is actually doing. Make the following connections from the ADALM2000 (M2K) to the circuit:
@@ Line 201: / Line 228: @@
 Set Scopy to Oscilloscope mode, with the following settings:
-  * Timebase: 200us/div
+  * Timebase: 250us/div
   * CH1, CH2: 1V/div
   * Triggering: Ch1, +5V, Rising Edge, Single Shot mode.
@@ Line 207: / Line 234: @@
 Momentarily short LT1054 pin 1 (FB) to ground. This disables the LT1054. Release FB; this allows the LT1054 to operate again, and produces a "clean" turn-on transient. You should see a waveform similar to the figure below:
-{{ :university:courses:electronics:sw_cap:lt1054_doubler_turn_on_scopy.png?600 |}}
+{{ :university:courses:electronics:sw_cap:lt1054_doubler_turn_on_scopy.png |}}
+<WRAP centeralign> Figure 23. LT1054 Doubler Startup Scopy Measurement</WRAP>
 Run the LTspice simulation, and probe the corresponding nodes. You should see results similar to the figure below:
 {{ :university:courses:electronics:sw_cap:lt1054_doubler_turn_on_ltspice.png?600 |}}
+<WRAP centeralign> Figure 24. LT1054 Doubler Startup LTspice Simulation</WRAP>
-Note that there is a noticeable qualitative difference between the Scopy measurement and the LTspice simulation; the measured rampup appears more linear, while the LTspice rampup appears more exponential.
+//Note that there is a noticeable qualitative difference between the Scopy measurement and the LTspice simulation; the measured rampup appears more linear, while the LTspice rampup appears more exponential.//
-Experiment 3: Voltage Divider
+====== TBD: Activity 3: Voltage Divider ======
-TBD: Experiment based on:
+Experiment based on:
 http://www.analog.com/media/en/technical-documentation/lt-journal-article/lt1054_0299_mag.pdf
-Construction:
-<<Measurements>>
 ====== Going Further: High-power switched capacitor circuits======
 Why are switched capacitor power circuits uncommon? High transient currents, EMI, etc. present challenges, but there are a number of practical, high-power switched-capacitor power converters. The LTC7820 is a fixed ratio high power charge pump controller - the evaluation board application circuit will halve the voltage of a 36V to 60V, at up to 10A (300W), at 99% efficiency.
 ===== Questions: =====
-Add questions here:
+<WRAP round download>
+**Lab Resources:**
+  * Fritzing files: [[downgit>education_tools/tree/master/m2k/fritzing/switch_cap_bb | switch_cap_bb]]
+  * LTSpice files: [[downgit>education_tools/tree/master/m2k/ltspice/switched_cap_ltspice| switch_cap_ltspice]]
+</WRAP>
 **Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]**

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