Analyzing a Damaging Voltage Cost Pump in LTspice—Supply and Load Resistance

Beforehand, I wrote an article that defined the fundamental slot gacor hari ini rules of damaging voltage, and I continued this theme with an LTspice lab that used simulations to elucidate damaging voltage as one thing that happens in and is produced by electrical circuits. As a part of this LTspice lab, I will additionally introduce a circuit topology that may produce a damaging voltage that’s steady and able to supplying present to different elements.

On this new sequence of articles, I want to take a extra detailed take a look at the performance of this damaging voltage circuit, with the target of enhancing our understanding of how a real-life switched-capacitor energy provide and energy provides, typically, could be optimized.

Recap: Damaging Voltage From Capacitors and Switches

Earlier than diving in, let’s take a look at Determine 1, which exhibits the cost pump circuit that I beforehand offered within the final damaging voltage article.

Determine 1. Instance cost pump circuit.

Within the circuit schematic, V1 produces the enter voltage, and V2 generates a 500 kHz sq. wave that controls all 4 switches. Resulting from the totally different resistance values assigned to the on and off states within the SW1 and SW2 fashions, S1 and S3 are on when S2 and S3 are off, and vice versa. The supply voltage costs capacitor C1 when S1 and S3 enable present movement, then all 4 switches change state, such that C1 discharges into the precise facet of the circuit.

Subsequent, C2 acquires a possible distinction equal to the V1 supply voltage, however for the reason that higher-voltage terminal of C2 is grounded, the lower-voltage terminal should shift into the damaging voltage area. Thus, the voltage on the INVERTED node is the same as damaging V(SOURCE). In different phrases, VOUT = –VIN.

The plot beneath (Determine 2) exhibits the output voltage pumping right down to after which remaining at –VIN.

Determine 2. Instance plot exhibiting the output voltage pumping right down to and remaining at –VIN.

Understanding the Impact of Load Resistance

Maybe you’re questioning if the switched-capacitor circuit is just too good to be true. Simply two capacitors, 4 switches, and a sq. wave? That’s all we have to generate a properly regulated damaging voltage provide rail? Properly, not fairly; this circuit isn’t truly a voltage regulator.

It’s not a voltage regulator as a result of it lacks one thing that’s central to the operation of each linear regulators and switch-mode regulators: a suggestions subsystem. Regulators keep steady, predictable provide voltages by monitoring the output and compensating for load variations by damaging suggestions.

Our switched-capacitor cost pump doesn’t have any type of damaging suggestions management system, and consequently, a lower in load resistance will trigger a corresponding lower within the output voltage. This happens as a result of the output community is basically a voltage divider. With that in thoughts, we have now the complete –VIN on the output when RLOAD = 100 kΩ solely as a result of 100 kΩ is far increased than the supply resistance (ROUT) of the cost pump. As RLOAD decreases towards ROUT, voltage is split extra equally between these two resistances, and thus the output voltage (i.e., the voltage throughout RLOAD) decreases.

You can too take into consideration this when it comes to load present. Let’s say that the operation of the load circuitry modifications such that the provision should ship extra present (that is electrically equal to a discount in RLOAD). When this happens, extra present flows by ROUT, extra voltage is dropped throughout ROUT, and a smaller proportion of the enter potential distinction is offered on the output node.

Simulating Variable Load Resistance in LTspice

We will use a .step textual content command, positioned proper onto the LTspice schematic, to visually assess the impact of various RLOAD:

.step PARAM LOAD record 100k 10k 1k 100 10 

This assertion will trigger the simulation to run as soon as for every worth within the record connected to the variable LOAD. We wish to assign these values to the RLOAD element, and we accomplish that through the use of LOAD (don’t overlook the curly brackets) within the component-value discipline (proven in Determine 3):

Determine 3. A bit of the cost pump circuit exhibiting the Rload LOAD.  

The consequence could be seen in Determine 4 beneath.

Determine 4. The ensuing simulation outcomes for the instance cost pump circuit.

The three highest resistance values (100 kΩ, 10 kΩ, 1 kΩ) all result in related efficiency, and the traces corresponding to those three values are virtually indistinguishable. Nonetheless, at 100 Ω (the beige hint), we start to note a lower in output voltage, and at 10 Ω (the inexperienced hint), the lower is reasonably extreme.

(I’m certain you additionally observed that voltage ripple will increase considerably as load resistance decreases. We’ll talk about that in Half 2.)

Evaluating Output Voltage for Utility Feasibility

Simulations like this one assist us to find out whether or not the circuit will keep satisfactory output voltage for a given software. Let’s say that we’d like a damaging voltage to energy a element with a provide requirement of –5 V ± 0.3 V; the minimal acceptable voltage magnitude, on this case, is 4.7 V. Utilizing our earlier outcomes as a place to begin, we create one other simulation (Determine 5) with RLOAD values that may convey us close to the related voltage threshold.

.step PARAM LOAD record 300 100 70 40

Determine 5.  Simulation outcomes the place the RLOAD brings us close to the related voltage threshold.   

Our outcomes recommend that the minimal secure RLOAD is a bit beneath 70 Ω. We’ll name it 65 Ω. A single-run simulation with RLOAD = 65 Ω confirms that we’re (theoretically) inside the acceptable vary, which could be seen beneath in Determine 6.

Determine 6. A single simulation consequence with RLOAD =  65 Ω.

Ohm’s regulation tells us that the load present with RLOAD = 65 Ω might be roughly 74 mA—you’ll be able to verify this through simulation in case you like. Thus, we conclude that if the whole load present is lower than 74 mA, the cost pump will be capable of keep satisfactory damaging provide voltage for the element in query.

All in all, we examined some fascinating particulars of an LTspice switched-capacitor cost pump, noting that the circuit isn’t a voltage regulator and utilizing .step simulations to find out load-current capabilities. Within the subsequent article, we’ll take a more in-depth take a look at output ripple.